ELEMENTARY   ELECTRO-TECHNICAL  SERIES 


'•ALTERNATING 
ELECTRIC    CURRENTS 


EDWIN  J.  HOUSTON,  Ph.D.,  (Princeton) 

AND 
A.  E.  KENNELLY,  So.  D. 


.    NEW  YOKE 

THE  W.  J.  JOHNSTON  COMPANY 
253  Broadway 
1895 


rK> 

yVS 


COPYEIGHT,     1895,     BY 

THE  W.  J.  JOHNSTON  COMPANY 


PREFACE 


IN  preparing  this  little  volume  on  Alter- 
nating Electric  Currents,  as  one  of  a  series 
entitled  the  Elementary  Electro -Technical 
Series,  the  authoi^ ^believe  that  they 
are  meeting  a  demand,  that  exists  on  the 
part  of  the  general  public,  for  reliable 
information  respecting  such  matters  in 
electrical  knowledge  as  can  be  readily  un- 
derstood by  those  not  specially  trained 
in  electro -technics. 

The  subject  of  alternating- electric  cur- 
rents is,  to-day,  perhaps,  the  most  promi- 
nent in  the  electrical  engineering  field. 
Although  when  profoundly  treated,  the 
subject  is  so  extremely  technical  as  not 
only  to  necessitate  the  use  of  advanced 
mathematics  but  also  to  require,  on  the 
part  of  the  student,  considerable  knowl- 
edge of  electricity,  yet  the  authors  feel 


tBEFACE 

confident  that  a  considerable  portion  of 
the  subject  can  readily  be  understood  by 
the  general  public.  They  therefore  offer 
this  volume,  with  the  belief  that  since 
the  commercial  applications  of  alterna- 
ting currents  are  rapidly  becoming  so  im- 
portant, it  is  no  longer  a  question  of 
willingness,  but  of  necessity,  on  the  part 
of  the  general  public,  to  become  familiar 
with  the  outlines  of  this  branch  of  elec- 
tro-technics. 


CONTENTS 

PAGE 

• 

I.     INTRODUCTORY 5 

II.     ALTERNATING  ELECTROMOTIVE 

FORCES  AND  CURRENTS      .  21 

III.  UNIPHASE  ALTERNATORS    .     .  53 

IV.  POWER 81 

V.     TRANSFORMERS     .     .     .     .     '  99 

VI.     ELECTRIC  LAMPS  .....  133 

VII.     ELECTRIC  MOTORS    ....  153 

VIII.       MULTIPHASED  CURRENTS     .       .  167 

IX.     MULTIPHASE  MOTORS    .     .     .  183 

INDEX  205 


/ALTERNATING 
ELECTEIC  CURRENTS. 


CHAPTER  I. 

INTRODUCTORY. 
O 

IN  a  river,  far  enough  above  its  mouth 
to  lie  beyond  the  reach  of  tidal  influences, 
the  water  constantly  flows  in  one  direc- 
tion; namely,  down  stream,  or  from  the 
source  toward  the  mouth.  Farther  down 
the  river,  within  the  tidal  limits,  the  di- 
rection of  flow  alternates,  or  is  reversed 
four  .times  in  about  twenty-four  hours: 
the  water  flowing  alternately  up  stream 


6          ALTERNATING  ELECTRIC!  CURRENTS. 

for  about  six  hours,  and  down  stream  for 
about  six  hours. 

In  continuous  electric  currents,  the  elec- 
tric flow  is  unidirectional;  i.e.,  takes  place 
continuously  in  one  direction  through  the 
conducting  channel,  like  a  river  above  the 
tideway.  In  alternating -electric  currents 
the  direction  of  flow  in  the  conducting 
circuit,  or  electric  channel,  is  alternately 
reversed,  like  a  river  within  the  limits  of 
tidal  influence. 

In  a  river,  the  current,  or  flow  of  water, 
changes  direction  but  four  times  in  every  24 
hours;  that  is,  during  this  time  there  are 
four  alternations  or  changes  of  direction. 
In  an  alternating- electric  circuit,  the  al- 
ternating-electric current,  or  flow  of  elec- 
tricity, changes  direction,  or  is  reversed, 
many  times  per  second.  The  number  of 


INTKODtJCTOfcY.  7 

alternations  per  second  is  commonly 
called  the  frequency  of  alternation.  In 
practice,  the  frequency  of  alternation  is 
from  50  to  270;  or,  in  other  words,  in  prac- 
tical alternating -current  circuits,  the  elec- 
tric current  makes  from  50  to  270  alterna- 
tions per  second,  according  to  the  system 
of  machinery  employed.  But  the  fre- 
quencies of  alternating  currents  may, 
under  certain  circumstances,  greatly  ex- 
ceed 270  alternations  per  second. 

In  the  case  of  telephonic  circuits,  over 
which  articulate  speech  is  transmitted,  al- 
ternating-electric currents  are  employed, 
the  frequency  of  which  may  be  1000 
or  more  alternations  per  second.  In  the 
experiments  of  Tesla,  in  which  special  ef- 
fects called  Tesla  effects  are  produced, 
extraordinarily  high  frequencies  are  em- 
ployed, reaching  sometimes  millions  of 
alternations  in  each  second  of  time. 


8          ALTEBNATING  ELECTBIC 


Kecent  investigations  have  shown  that 
light  is,  in  all  probability,  an  effect  pro- 
duced in  space  by  alternating  -electric 
currents  of  frequencies  reaching  as  high 
as  800  trillions  per  second. 

In  the  case  of  a  tidal  stream,  the  time 
required  for  the  flow  of  water  to  return  to 
the  condition  it  had  at  any  moment,  may 
be  called  the  period  of  the  stream.  Thus, 
suppose  a  river  at  high  water  is  just  be- 
ginning to  ebb;  then  a  period  will  include 
the  time  required  to  again  reach  high 
water,  and  will  embrace  the  time  of  one 
full  ebb  and  one  full  flood;  in  this  case, 
about  12  hours.  During  one  period  the 
flow  of  water  in  the  river  will  have  com- 
pleted one  cycle,  and  will  have  undergone 
two  alternations,  or  reversals  of  direction. 
Every  complete  cycle,  therefore,  consists 
of  two  alternations.  In  the  case  of  the 


INTRODUCTORY.  9 

river,  the  duration  of  ebb  and  flood  are  un- 
equal. In  the  case  of  all  practical  alter- 
nating currents,  the  duration  of  each  re- 
versal or  alternation  is  the  same. 

The  period  of  an  alternating -electric 
current  is  the  time  required  to  complete 
two  alternations,  or,  in  other  words,  to 
effect  one  complete  cycle.  The  number  of 
cycles  per  second  is  called  the  frequency. 
The  time  occupied  in  each  reversal  is 
sometimes  called  a  semi-period.  Conse- 
quently, an  electric  current,  making  100 
reversals  or  alternations  per  second, 
would  have  a  frequency  of  100  alterna- 
tions, or  50  complete  cycles,  per  second. 

In  the  case  of  most  tidal  streams,  the 
water  rises  or  falls  at  a  comparatively  uni- 
form rate;  that  is,  if  the  range  of  the  tide 
is  six  feet,  and  the  difference  of  level  pro- 


10       ALTERNATING  ELECTRIC  CURRENTS. 

duced  during  ebb  or  flood  is  rigorously 
one  foot  per  hour,  then  the  level  of  the 
water  in  the  river,  at  any  time,  might  be 
graphically  represented  as  in  Fig.  1,  where 
we  assume  that  at  noon,  each  day,  high 
water  occurs  three  feet  above  the  mean 
level;  at  3  P.  M.  the  mean  sea  level  is 
reached;  at  6  P.  M.,  low  water;  at  9  p.  M., 

HIGH  WATER 


_  LOW  WATER 

FIG.  1.— TIDAL  FLOW  OF  RIVER. 

mean  level,  and  at  midnight,  high .  water, 
completing  the  cycle  in  a  period  of  12 
hours.  In  this  ideal  case,  the  water  is 
flowing  from  noon  to  6  p.  M.  and  from 
midnight  to  6  A.  M.  out  of  the  river,  at  a 
steady  rate,  of  say  500,000  gallons  per 
hour,  and  is  flowing,  at  the  same  rate,  from 
6  P.  M.  to  midnight,  and  from  6  A.  M.  to 


INTRODUCTORY. 


11 


noon,  steadily  back  into  the  river.  If, 
therefore,  it  be  required  to  represent  the 
rate -of -flow  of  the  river,  that  is,  the  quan- 
tity of  water  passing  per  hour,  or  per  sec- 
ond, it  will  be  necessary  to  employ  a  new 
diagram,  such  as  that  shown  in  Fig.  2. 
Here  distances  above  the  line  0  0,  corre- 


.   100,00 

O  FLOW 

100,00 

200,00 

soo,oo 


FULL  FLOW  UPSTREAM       FULL  FLOW  UP8TRB 

'"I 

4 

*"""  (0 

—       K 

i 

.  —  i 

FULL  FLOW  DOWNSTREAM     FULL  FLOW  DOWNSTREAM 

FIG.  2. — CURVE  OF  TIDAL  FLOW. 

spond  to  flood  tide,  or  flow  up  stream, 
and  distances  below  the  line,  correspond 
similarly  to  ebb  tide,  or  flow  down  stream. 
Thus,  between  noon  and  6  p.  M.,  500,000 
gallons  per  hour,  or  nearly  140  gallons 
per  second,  flow  steadily  down  stream 
toward  the  mouth,  while  from  6  P.M.  to  12 
midnight,  there  is  the  same  flow  up  stream. 


12       ALTERNATING  ELECTRIC  CURRENTS. 

If  the  above  diagrams  represented  the 
actual  condition  of  affairs,  high  water  and 
low  water  could  only  exist  for  an  infinites!  - 
mally  small  interval  of  time,  whereas,  we 
know  that  slack  water  has  an  appreciable 
duration,  and  that  the  rate  of  rising  or 
falling  is  not  uniform,  but  is  greatest  about 


FIG.  3.—  TIDAL  LEVEL  OF  RIVER. 

mean  tide.  This  is  represented  for  the 
ideal  case  of  a  12  -hour  period  and  a  uni- 
form tide,  in  Fig.  3,  and  the  flow  diagram 
in  Fig.  4,  corresponding  to  Fig.  3,  shows 
that  the  rate  -of  -flow,  instead  of  changing 
direction  abruptly,  does  so  gradually,  so 
that  instead  of  the  rectangular  wave  of 
Fig.  2,  we  have  a  smooth  wave. 


INTEODUCTOEY. 


13 


Figs.  2  and  4  may  also  be  taken  to  rep- 
resent alternating -electric  current  flow  as 
well  as  alternating  tidal  flow,  except  that 
a  period  would  then  correspond  to  but  a 
fraction  of  a  second,  instead  of  approxi- 
mately 12  hours,  and  the  rate-of-flow 


FIG.  4.—  CURVE  OF  TIDAL  FLOW. 

would  be  measured  or  marked  off,  not  in 
#a//ons-per-hour,  but  in  units  of  electrical 
flow  called 


Fig.  5  is  a  reproduction  of  Fig.  2,  except 
that  the  period  is  1  -100th  of  a  second, 
corresponding  to  an  electrical  frequency  of 


14       ALTEBNATING  ELECTBtC  CIJBBENTS. 


100  cycles,  or  200  alternations  per  second; 
while  the  flow  is  alternately,  say  50  cou- 
lombs of  electricity  per  second  in  one 
direction,  and  then  50  coulombs -per- sec- 
ond in  the  opposite  direction. 

Ill 

m  5*VJ 


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1 

I 

§ 

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*~                              10 

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* 

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•* 

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I  ( 
•A* 

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Hi  >    "  i 

FIG.  5. — CURVE  OF  ALTERNATING-CURRENT  FLOW. 

A  coulomb-per-second,  considered  as  a 
rate  of  flow,  is  called  an  ampere.  Instead, 
therefore,  of  using  the  phrase  coulomb- 
per-second,  we  may  use  the  word  ampere. 


INTRODUCTORY*  15 

The  current  strength,  or  flow,  represent- 
ed by  Fig.  5,  is  alternately  50  amperes 
in  one  direction  and  50  amperes  in  the 
opposite  direction  throughout  all  parts  of 
the  conducting  circuit. 

In  an  alternating-current  circuit,  that  is, 
in  a  complete  conducting  path  through 
which  alternating -electric  currents  may 
flow,  the  current  strength,  at  any  instant, 
as  expressed  in  amperes,  is  the  same  at  all 
parts  of  the  circuit,  so  that  if  the  current 
strength  be  50  amperes  in  one  direction, 
it  will,  as  a  rule,  at  that  moment,  be  50 
amperes  in  that  direction  throughout  the 
circuit,  and,  when  the  reversal  takes 
place,  it  will  practically  do  so  coincideni.- 
ly  throughout  the.  ckciiiL.and  the  current 

1  ""  "™"         •' 

strength  becomes,  as  is  seen  in  Fig.  5,  50 
amperes  in  the  opposite  direction  in  all 
parts  of  the  circuit. 


16       ALTERNATING  ELECTRIC  CURRENTS. 

Fig.  6  is  practically  a  reproduction  of 
Fig.  4,  and  represents  an  alternating  cur- 
rent with  a  frequency  of  50  cycles,  or  100 


FIG.  6.— CURVE  OF  ALTERNATING-CURRENT  FLOW. 

alternations  per  second,  and  a  maximum 
strength  of  20  amperes  in  each  alternation. 
The  condition  of  things  represented  in  Fig. 
6,  is  a  much  closer  approximation  to  the 
actual  state  of  most  commercial  alterna- 
ting-current circuits  than  that  represented 
in  Fig.  4,  since,  in  fact,  the  electric  cur- 


INTRODUCTORY.  17 

rent  can  never  change  instantaneously 
from  a  full  positive  to  a  full  negative 
strength,  or  vice-versa,  but  usually  fol- 
lows some  smooth  curve. 

For  convenience,  we  have  compared  the 
flow  of  water  through  a  river  channel  with 
the  flow  of  electricity  through  a  conduct- 
ing channel  or  circuit.  We  should,  how- 
ever, carefully  avoid  falling  into  the  error 
of  carrying  this  analogy  too  far,  since 
electricity  is  not  a  fluid,  although  many  of 
the  laws  of  its  passage  and  flow  bear  close 
resemblance  to  the  laws  of  liquid  flow. 

Although,  at  the  present  time,  the  exact 
nature  of  electricity  is  far  from  being 
known,  yet  electricity  is  generally  believed 
to  be  an  effect  produced  by  an  active  con- 
dition in  an  all-pervading  medium  called 
the  ether.  The  ether  is  believed  to  fill  in- 


18        ALTERNATING  ELECTRIC  CURRENTS. 

terstellar  space  and  to  permeate  all  bod- 
ies, even  copper  wires,  and  other  equally 
dense  forms  of  matter.  Just  what  may 
be  the  nature  of  that  particular  ether  ac- 
tivity which  constitutes  electricity,  is  not 
known.  It  may  or  may  not  resemble  the 
particular  form  of  activity  in  the  atmos- 
phere called  whirlwind. 

The  difficulty  of  obtaining  a  clear  con- 
ception of  the  true  nature  of  electricity 
arises  from  our  inability  to  recognize  even 
the  existence  of  the  ether  by  our  senses, 
and  our  still  greater  inability  to  recognize 
the  conditions  of  its  activity.  In  the  case 
of  the  atmosphere,  we  can  readily  appreci- 
ate the  phenomena  produced  by  the  wind, 
since  the  effects  are  produced  on  a  scale 
commensurate  with  the  capabilities  of  our 
senses.  But,  were  we  situated  on  a  dis- 
tant planet,  and  had  no  experience  what- 


INTEODUCTOEY.  19 

ever  of  an  atmosphere,  even  though  we 
could  perceive,  through  sufficiently  pow- 
erful glasses,  the  effects  of  storms  on  the 
earth,  we  would,  probably,  have  as  great 
difficulty  in  understanding  the  nature  of 
phenomena  produced  by  wind  power,  as 
we  now  have  in  understanding  the  nature 
of  electrical  phenomena,  as  possible  ef- 
fects of  ether  disturbance. 

The  researches  of  the  eighteenth  cent- 
ury gave  rise  to  the  belief  that  electricity 
was  a  subtle  fluid  to  which  the  name  of 
electric  fluid  was  given.  The  researches 
of  the  nineteenth  century  have  promoted 
the  belief  that  this  fluid  is  no  other  than  the 
all-pervading  ether  which  serves  to  con- 
vey  over  apparently  empty  spaces  heat, 
light,  gravitational  force,  and  magnetism. 
Certain  characters  of  disturbance  in  this 
medium  produce  phenomena  which  we 
recognize  as  electrical,  while  other  dig- 


20       ALTERNATING  ELECTEIC  CUEEENTS. 

turbances  of  a  distinct  but  interconnected 
character  with  the  preceding,  give  rise 
to  phenomena  which  we  recognize  as 
magnetic. 


CHAPTER  II. 

ALTERNATING  ELECTEOMOTIVE  FORCES  AND 
CURRENTS. 

IN  all  commercial  applications  of  elec- 
tricity the  following  combinations  of  parts 
are  needed;  namely, 

(1)  A  device  called  a  source,  where  the 
electric  current  originates. 

(2)  Devices  called  translating  or  recep- 
tive devices. 

(3)  Conducting    paths    connecting    the 
translating     devices    with   the     electric 
source. 

In  all  cases,  after  an  electric  current  has 
left  its  source  and  produced  some  peculiar 
effect  in  a  receptive  device,  placed  in  its 
path  or  circuit,  means  must  be  provided 


22        ALTERNATING  ELECTRIC  CURRENTS. 

whereby  the  current  may  flow  back  again 
to  the  source.  In  other  words,  the  elec- 
tricity invariably  leaves  the  source,  passes 
through  various  conducting  paths,  pro- 
duces effects  in  the  translating  devices, 
and  flows  back  to  the  source  from  which 
it  came.  For  this  reason,  the  conducting 
path  is  usually  called  a  circuit,  although 
of  course  it  is  not  necessary  that  the  path 
through  which  the  electricity  flows  should 
be  a  circular  path. 

Electric  sources  do  not  primarily  pro- 
duce electricity,  but  a  particular  variety 
of  force  called  electromotive  force,  (general- 
ly abbreviated  E.  M,  F.  ).  This  force,  in 
its  turn,  tends  to  produce  electric  current, 
In  point  of  fact,  an  electric  source,  al- 
though it  will  always  produce  electro- 
motive force  in  a  conducting  circuit  con- 
nected to  it,  yet  will  not  produce  an  elec- 


ELECTEOMOTIVE  FOKCES.  23 

trie  current  in  such  circuit,  unless  the 
circuit  be  closed  or  completed. 

Electromotive  forces  are  either  contin- 
uous or  alternating.  A  continuous  elec- 
tromotive force  is  unidirectional;  i.  e.,  has 
continuously  the  same  direction,  and  pro- 
duces, when  it  acts  upon  a  closed  circuit, 
what  is  called  a  continuous  electric  current. 
An  alternating  electromotive  force  is  one 
which  alternates  in  direction,  and,  when 
applied  to  an  electric  circuit,  produces  an 
alternating  electric  current;  that  is,  an 
electric  .current,  the  direction  of 
periodically  changes  with  the  ch 
the  direction  of  the  E.  M.  F. 


A  voltaic  cell  is  an  example  of  an  elec- 
tric source  which  produces  a  continuous 
electromotive  force.  A  common  and  con- 
venient form  of  voltaic  cell,  much  em- 
ployed on  telegraph  lines,  is  called  the 


24       ALTERNATING  ELECTEIC  CURRENTS. 

Daniell  Gravity  Cell.  Such  a  cell  is  shown 
in  Fig.  7.  It  consists  of  a  plate  of  copper 
C9  and  a  plate  of  zinc  Zn,  immersed  re- 
spectively in  aqueous  solutions  of  copper 

JL 


FIG.  7. — GRAVITY  CELL. 

sulphate  and  zinc  sulphate.  A  solution 
of  zinc  sulphate  will  float  on  a  solution  of 
copper  sulphate,  being  lighter  than  it,  and 
since  this  fact  is  utilized  to  keep  the  liq- 
uids separated,  the  form  of  cell  in  which 
the  solutions  are  thus  separated,  is  called 
the  gravity  cell. 


ELECTROMOTIVE  FORCES. 


25 


The  current  produced  is  conventionally 
assumed  to  leave  the  cell  at  its  positive  or 
copper  pole,  and  to  return  to  it,  after  hav- 
ing passed  through  the  conducting  circuit, 
and  its  receptive  device,  at  its  negative  or 
zinc  pole.  When  the  terminals  of  the  cell 


FIG.  8. — ILLUSTRATING  REVERSAL  IN  DIRECTION  OF  CURRENT 
THROUGH  AN  ELECTRIC  CIRCUIT  ON  THE  REVERSAL,  OF 
ITS  ELECTROMOTIVE  FORCE. 

are  connected  to  a  circuit,  a  current  will 
flow  through  the  external  circuit  from  the 
copper  pole  to  the  zinc  pole,  as  shown  in 
Fig.  8.  But  if  the  terminals  of  the  cell 
be  reversed,  the  direction  of  the  flow 
through  the  conductor  will  be  reversed, 


26       ALTEKNATING  ELECTBIC  CURRENTS. 

and,  if  these  reversals  are  made  five  times 
per  second,  then  there  will  be  five  alter- 
nations of  electromotive  force  and  current 
in  the  circuit  per  second.  The  alternating 
currents  employed  in  practice,  are  not, 
however,  obtained  in  this  way,  but  from 
special  machines  called  alternators. 

In  its  action  on  an  electric  circuit,  a 
continuous  electromotive  force  resembles 
the  action  of  a  watermotive  force,  or  pres- 
sure in  a  reservoir,  which  forces  a  steady 
stream  of  water  through  an  outflow  pipe. 
An  alternating  electromotive  force  resem- 
bles in  its  action  the  action  of  an  alterna- 
ting water  motive  force,  or  pump,  alternately 
pumping  water  into  and  out  of -a  reservoir 
through  a  pipe.  Water  engines,  operated 
by  water  pressure  alternately  exerted 
on  opposite  sides  of  a  piston,  after  the 
general  manner  of  the  action  of  a  steam 


ELECTROMOTIVE  FORCES.  27 

engine,  afford  an  instance  of  such  an  al- 
ternating watermotive  force. 

When  a  continuous  electromotive  force 
is  applied  to  a  conducting  circuit,  such, 
for  example,  as  a  mile  of  insulated  cop- 
per wire,  the  current  which  passes  through 
the  circuit  will  be  twice  as  great  as  it 
would  be,  if  the  same  E.  M.  F.  were  ap- 
plied to  a  circuit  of  the  same  length  of 
such  wire,  but  of  only  half  the  weight  or 
area  of  cross -section;  for,  the  thicker  wire 
conducts  electricity  twice  as  well  as  the 
thinner  wire;  or, mother  words, offers  but 
one-half  the  resistance. 

Electrical  resistance  is  usually  ex- 
pressed in  units  called  ohms.  The  ohm  is 
the  resistance  offered  by  a  given  length 
of  conductor  of  definite  cross -section. 
When  the  resistance  of  any  circuit  is 


28        ALTERNATING  ELECTBIC  CUBBENTS. 

known  in  ohms,  the  current,  produced  by 
applying  to  this  circuit  a  known  E.  M. 
F.,  can  be  calculated  in  amperes,  by  a 
rule  called  Ohiris  law,  from  the  name  of 
its  discoverer,  Dr.  Ohm,  of  Berlin. 

Ohm's  law  is  usually  expressed  as  fol- 
lows: 

The  current  in  any  conducting  circuit,  ex- 
pressed in  amperes,  is  equal  to  the  total  elec- 
tromotive force  in  the  circuit,  expressed  in 
volts,  divided  by  the  resistance  of  the  circuit, 
expressed  in  ohms. 

In  other  words,  the  amperes  in  any 
circuit  are  equal  to  the  volts  divided  by 
the  ohms.  Thus,  the  electromotive  force 
usually  supplied  to  incandescent  electric 
lamps  is  about  110  volts,  and  since  the 
resistance  of  the  carbon  filament  in  a 
sixteen-candle  power  lamp,  when  lighted, 
is,  say  220  ohms,  the  current  strength, 


ELECTKOMOTIVE   FORCES.  29 

which  will  pass  through  such  a  lamp,  is 
110  volts  -4-  220  ohms  =  1-2  ampere, 

If  the  electric  resistance  of  any  insu- 
lated wire  be  measured  in  ohms,  the 
value  will  be  found  to  be  the  same, 
whether  the  wire  be  straight  or  bent; 
i.e.,  whether  the  wire  be  stretched  in  a 
straight  line,  or  be  wrapped  in  a  close  coil; 
for,  when  a  continuous  current  is  once 
established  in  a  wire  or  conductor,  bends 
or  turns  in  the  direction  of  the  conductor 
do  not  offer  any  additional  resistance  to 
the  flow  of  the  current.  When,  however, 
an  alternating  electromotive  force  is  ap- 
plied to  a  wire,  the  strength  of  the  current 
established  in  the  circuit  is  considerably 
influenced  by  the  disposition  of  the  wire, 
that  is,  whether  it  forms  a  single  loop, 
or  whether  it  forms  a  coil  of  many  turns. 
In  the  latter  case,  the  current  which 


30       ALTERNATING  ELECTEIC  CUEEENTS. 

flows  is  much  smaller  than  that  obtained 
by  dividing  the  E.  M.  F.  in  volts,  by  the 
resistance  of  the  coil  in  ohms.  In  other 
words,  a  different  law  appears  to  govern 
the  current  strength  in  an  alternating-cur- 
rent circuit  than  that  which  governs  it  in 
a  continuous -current  circuit.  A  circuit 
containing  coils  of  wire,  acts  toward  an 
alternating  E.  M.  F.  as  if  it  possessed  a 
higher  resistance  than  when  traversed  by 
a  steady  current.  In  other  words,  the 
passage  of  an  alternating  current  through 
a  coil  of  wire  is  opposed  by  an  influence 
which  tends  to  choke  or  diminish  the 
current.  This  influence  is  called  the  re- 
actance of  the  coil.  The  nature  of  react- 
ance will  be  understood  from  a  consider- 
ation of  the  following  principles:  When 
an  electric  current  is  sent  through  a  con- 
ductor, the  conductor  thereby  acquires 
all  the  properties  of  a  magnet,  as  was  first 


ELECTROMOTIVE  FO&CES.  31 

shown  by  Oersted,  in  1819.  Could  we 
see  the  actual  state  of  things  which 
exists  in  the  neighborhood  of  an  active 
conductor,  it  is  believed  -  that  we  would 
be  able  to  see  around  the  conductor,  a 
streaming  motion  in  concentric  circu- 
lar paths,  of  the  highly  tenuous,  all -per- 
vading medium,  called  the  ether. 

The  ether  streaming  motion  is  called 
magnetism.  It  is  most  energetic  in  the 
immediate  neighborhood  of  the  conductor, 
gradually  becoming  weaker  at  greater  dis- 
tances from  it.  Moreover,  the  direction  of 
the  streaming  depends  upon  the  direction 
of  the  current  in  the  conductor.  For  ex- 
ample, if,  as  in  Fig.  9,  the  current  passes 
downward"~through  the  plane  of  the  pa- 
per, that  is,  from  the  observer,  the  direc- 
tion of  the  streamings  will  be  the  same 
as  the  direction  of  the  hands  of  a  watch. 


32        ALTERNATING  ELECTRIC  CURRENTS. 

These  ether  streamings  occur  in  the 
space  around  every  magnet,  as  well  as  in 
the  space  around  an  active  conductor,  and 
constitute  what  is  called  a  magnetic  field. 

If  the  conductor  be  given  the  form  of  a 


' 


From  Observer  Toward  Observer 

FIG.  9.—  DIAGRAMS  OF  FLUX  PATHS  ROUND  A  WIRE  CARRY- 
ING A  CURRENT  FROM  AND  TOWARD  OBSERVER. 

loop  and  the  ends  of  the  loop  be  connect- 
ed with  an  electric  source,  so  that  an  elec- 

tric current  flows  through  the  circuit  so 
formed,  then  the  ether  streamings,  or  the 
magnetic  flux  surrounding  the  wire,  will 
be  so  directed  that  all  the  flux  will  enter 


ELECTROMOTIVE   FORCES.  66 

the  loop  at  one  side  and  leave  it  at  the 
opposite  side.  The  only  effect  produced 
by  changing  the  direction  of  the  current, 
will  be  to  change  the  direction  in  which 
the  flux  passes  through,  or  threads  the 
loop.  If,  for  example,  with  one  direction 
of  current  flowing  through  the  conducting 
loop,  the  magnetic  flux  enters  the  loop 
from  above  and  passes  out  below,  then 
reversing  by  the  direction  of  the  electric 
current,  the  flux  would  enter  the  loop 
from  below  and  pass  out  from  above. 

The  effect  of  impressing  any  E.  M.  F.  on 
a  conducting  loop  is,  therefore,  to  cause 
magnetic  flux  to  thread  or  pass  through 
the  loop.  Conversely,  the  effect  of  causing 
magnetic  flux  to  pass  through  a  loop  is  to 
produce  an  E.  M.  F.  in  the  loop.  This 
E.  M  .  F  .  continues  only  while  the  flux 
passing  through  the  loop  is  changing  in 


34        ALTERNATING  ELECTRIC  CURRENTS. 

amount;  or,  in  other  words,  while  it  is  in- 
creasing or  decreasing.  An  E.  M.  F.  set 
up  in  this  manner  in  a  conducting  loop  is 
called  an  induced  E.  M.  F.  The  direction 
of  the  induced  E.  M.  F.  is  opposite  to  the 
direction  of  the  E.  M.  F.  which  was  re- 
quired to  produce  the  flux  that  caused  it. 
In  order  to  distinguish  the  E.  M.  F.  pro- 
ducing the  flux,  from  the  E.  M.  F.  pro- 
duced by  the  flux,  the  former  is  called  the 
impressed  E.  M  .  F  .  In  other  words,  the 
passage  of  magnetic  flux  through  a  con- 
ducting loop,  consequent  upon  the  appli- 
cation of  an  E.  M.  F.  to  such  loop,  will  tend 
to  set  up  in  the  loop  an  E.  M.  F.  oppo- 
sitely directed  to  that  of  the  impressed 
E.  M.  F.  The  induced  E.  M.  F.  is,  conse- 
quently, called  a  counter  electromotive 
force;  and,  since  it  is  produced  by  induc- 
tion, it  is  sometimes  called  the  counter 
electromotive  force  of  self-induction. 


ELECTROMOTIVE  FORCES.  35 

The  intensity  of  the  counter  E.  M.  F.  so 
set  up,  depends  upon  the  rate  of  change  in| 
the  amount  of  flux  passing  through  the  j 
loop  at  any  moment,  and  not  on  the  total 
amount  of  flux.     Consequently,  when  the 
direction  of  current  is  reversed,  as  in  an 
alternating -current  circuit,  the  direction 
of  the  flux  is  reversed,  and  a  rapid  change 
occurs  in  the  rate  at  which  the  flux  is 
passing  through  the  loop. 

The  effect,  therefore,  of  applying  an  al- 
ternating E.  M.  F.  to  a  coil  of  wire  is  to 
produce,  by  induction,  a  resistance  to  cur- 
rent flow  greater  than  the  resistance  to 
steady  currents.  This  total  apparent  re- 
sistance, which  is  generally  called  imped- 
ance, arises  from  the  fact  that  the  rapid 
filling  and  emptying  of  the  coils  with  mag- 
netic flux,  set  up  an  E.  M.  F.  counter  or 
opposed  to  the  E.  M,  F.  driving  the  flux 


36       ALTEKNATING  ELECTRIC  CURRENTS. 

through  the  coils,  and,  therefore,  impedes 
the  flow  of  current  through  the  coils. 
The  effect  of  the  impedance  is  to  prevent 
the  immediate  application  of  Ohm's  law 
to  an  alternating-current  circuit. 

The  resistance  of  100  feet  of  insulated 
copper  wire  of  the  size  represented  in  Fig. 
10,  and  which  is  known  commercially  as 


FIG.  10.— No.  13,  A.  W.  Gr.  WIRE,  FULL  SIZE. 

No.  13,  American  Wire  Gauge,  cotitracte'd 
A.W.G.  is  approximately  l-5th  of  an  ohm. 
If  a  continuous  E.  M.  F.  of  one  volt  be 
maintained  between  the  ends  of  this  wire, 
the  current  strength  through  the  wire, 
whether  straight  or  wound  into  a  coil, 
would,  by  Ohm's  law,  be  five  amperes  (1 
volt  -s-  l-5th  ohm  =  5  amperes).  But  if  an 
alternating  E.  M.  F.  of  one  volt,  reversing 


ELECTROMOTIVE   FORCES.  37 

250  times  a  second,  and,  therefore,  having 
a  frequency  of  250  reversals,  or  125  cycles 
per  second,  be  connected  to  the  ends  of  the 
wire,  the  current  strength  through  the 
wire,  if  the  wire  be  wound  into  a  coil  of 
many  turns,  will  be  considerably  reduced, 
say  to  2  amperes,  and  the  impedance,  or 
apparent  resistance  of  the  wire,  will  be  1-2 
ohm,  instead  of  l-5th  ohm. 

f?> 

The  impedance  increases  both  with 
frequency  and  with  the  number  of  turns 
in  the  coil.  But,  as  we  have  already 
seen,  a  counter  E.  M.  F.  is  produced  in  a 
coil  by  a  change  of  flux  passing  through 
the  coil.  The  effect  of  introducing  iron 
into  the  path  of  the  magnetic  flux,  is  to 
increase  the  amount  of  flux  which  passes, 
owing  to  the  fact  that  iron  conducts  mag- 
netic flux  much  better  than  air.  If,  then, 
a  coil  of  wire  be  wound  on  a  suitable  core 


38        ALTERNATING  ELECTRIC  CURRENTS. 

of  iron,  the  flux  passing  through  the  coil, 
at  each  reversal  of  current,  will  be  great- 
ly increased,  and,  consequently,  the  reac- 
tance of  the  coil  will  be  increased,  or  the 
coil  will  possess  a  greater  impedance  and 
a  more  marked  choking  effect,  when  the 
core  is  present,  than  when  it  is  absent. 

It  might  be  supposed  that  alternating  - 
electric  currents  possess  a  marked  dis- 
advantage over  continuous  currents  from 
the  fact  that  the  introduction  of  coils  of 
wire  into  their  circuit  necessarily  tends  to 
impede  or  choke  the  current  flow;  for,  as 
is  well  known,  nearly  all  electric  appara- 
tus contain  coils  of  wire,  as,  for  example, 
electromagnets,  But  this  very  fact,  so  far 
from  being  an  unmitigated  detriment,  is 
often  employed  to  great  advantage,  where 
the  amount  of  current  which  can  flow 
through  a  circuit  is  automatically  choked 


ELECTROMOTIVE    FORCES.  39 

or  throttled  by  the  impedance  of  coils  of 
insulated  wire.  In  fact  the  capability  of 
introducing  reactance,  practically  without 
resistance,  into  an  alternating  current  cir- 
cuit, is  one  of  the  principal  advantages  of 
alternating  currents. 

It  is  true  that  an  electric  current,  wheth- 
er continuous  or  alternating,  can  be  read- 
ily diminished  in  strength  by  the  intro- 
duction into  the  circuit  of  mere  resist-' 
ance,  called  ohmic  resistance,  because  its 
resistance  depends  only  on  the  nature  of 
the  wire,  its  length  and  area  of  cross-sec- 
tion, and  is  independant  of  the  disposi- 
tion of  the  wire,  or  its  coiling.  But,  in 
the  case  of  an  alternating  current,  the 
counter  E,  M.  F.  prevents  a  portion  of 
the  electromotive  force  from  acting  and, 
therefore,  decreases  the  amount  of  elec- 
trical work  done,  or  energy  usefully  ex- 


40       ALTERNATING  ELECTRIC  CUEEENTS. 

pended,  while  with  the  continuous  cur- 
rent, although  the  current  is  reduced, 
yet  the  entire  E.  M.  F.  is  acting  and,  con- 
sequently, there  is  a  greater  expendi- 
ture of  power. 

An  application  of  the  methods  of  vary- 
ing, in  certain  cases,  the  strength  of  cur- 
rent flowing  through  any  circuit,  is  seen 
in  the  solution  of  a  problem,  which  is 
often  met  in  practice;  namely,  to  turn 
down  or  decrease  the  brightness  of  an 
electric  lamp.  If  this  be  done,  as  has  fre- 
quently been  attempted,  by  introducing 
into  the  circuit  of  the  lamp,  a  mere  ohmic 
resistance;  namely,  a  conductor  with  but  a 
few  turns,  then,  although  the  strength  of 
current  passing  through  the  lamp  is  de- 
creased, and  power  saved  in  this  respect, 
yet  the  same  current  is  now  passing 
through  the  resistance  and  producing  use- 


ELECTROMOTIVE  FORCES.  41 

less  heat  in  it.  On  the  contrary,  when  a 
reactance,  i.  e.,  a  coil  of  many  turns,  is 
employed  with  an  alternating  current,  not 
only  is  the  current  passing  through  the 
lamp  decreased,  but  practically  no  energy 
is  lost  in  the  reactance. 

Fig.  11  represents  a  form  of  device  for 
turning  down  lights,  called  a  theatre  dim- 
mer. Here  a  portion  of  the  circuit  con- 
taining the  lamps  is  wrapped  in  the  form 
of  a  coil  C\  around  a  laminated  ring  of 
soft  iron  K;  that  is,  a  ring  consisting  of 
plates  of  soft  sheet  iron,  laid  side  by  side. 
On  the  opposite  side  of  the  soft  iron  ring 
/C  a  copper  shield  H,  is  placed,  capable 
of  being  slid  over  the  core  K,  to  the  right 
or  the  left  about  the  axis  Z>,  by  the  mo- 
tion of  the  hand  wheel.  With  the  rela- 
tive positions  occupied  by  the  shield  H, 
and  the  coil  C9  shown  in  the  figure,  the 


42        ALTEKNATING  ELECTRIC  CURRENTS. 


effect  of  the  coil  is  to  throttle,  or  choke, 
the  current,  by  its  reactance,  and  thus  di- 
minish the  intensity  of  the  light  given  by 
the  lamps.  If  it  be  desired  to  increase 
the  amount  of  light,  that  is,  to  turn  the 


^  FROM  DYNAMO 


FIG.  11.— THEATRE  DIMMER,  REACTIVE  COIL. 

lights  up,  the  metal  shield  H,  is  moved 
by  the  hand  wheel  toward  the  reactive 


ELECTROMOTIVE    FORCES. 


43 


coil  C,  thereby  diminishing  the  reactance 
of  the  coil,  and  thus  permitting  more  cur- 


FIG.  12.— THEATRE  DIMMER. 

rent  to  flow  through  the  circuit.  A  mo- 
tion, therefore,  of  the  metal  shield  H, 
toward  C,  increases  the  intensity  of  the 
light,  while  a  motion  from  C,  diminishes 
the  intensity.  A  perspective  view  of  the 
apparatus  is  shown  inFig.12.  Fig.13  shows 
other  forms  of  theatre  dimmer,  which 
operate  by  the  choking  effect  of  react- 


44        ALTERNATING  ELECTRIC  CURRENTS. 

ive  coils  furnished  with  a  movable  core 
consisting  of  a  bundle  of  soft  iron  wires. 


^  FIG.  13— ALTERNATING  CURRENT  THEATRE  DIMMERS. 

Both  continuous  and  alternating   cur- 
rents are  capable,  when  passed  through 


ELECTROMOTIVE  FORCES.  45 

coils  of  insulated  wire  provided  with  iron 
cores,  of  producing  electromagnets  as 
shown  in  Fig.  14.  Continuous -electric 
currents  are  generally  employed  for  this 
purpose,  since  the  magnetizing  coils  do 
not  then  act  to  throttle  the  current. 
When  alternating- electric  currents  are 
passed  through  the  coils  of  an  electromag- 


FIG.  14.— FORM  OF  ELECTROMAGNET. 

net,  although  such  a  magnet  does  not  pos- 
sess as  powerful  attraction  for  its  arma- 
ture, as  when  excited  by  continuous  cur- 
rents, yet  it  often  possesses  the  advantage 
of  exerting  a  more  nearly  uniform  puj.1 
over  a  greater  distance.  Of  course,  in 
alternating -current  electromagnets,  the 


46        ALTEENATING  ELECTEIC  CUEEENTS. 

magnetism  is  constantly  reversing  in  di- 
rection, with  each  reversal  of  the  current, 
each  pole  becoming  alternately  of  north 
and  south  polarity. 

In  electroplating,  deposits  of  gold,  sil- 
ver and  other  metals  are  thrown  down  by 
the  action  of  an  electric  current  on  the 
conducting  surfaces  of  articles  placed  in 
suitable  vats.  The  surfaces  which  are  to 
receive  these  deposits,  if  not  already  con- 
ducting, are  made  so  by  various  processes, 
and  immersed  in  solutions  of  the  metals 
with  which  they  are  to  be  coated.  The 
current  employed  for  this  purpose  is  invar- 
iably a  continuous  current.  It  is  a  well- 
known  fact,  that  an  article,  which  has 
been  placed  in  a  plating  bath  and  has  re- 
ceived a  coating  of  deposited  metal  by  the 
electric  current  passing  through  the  bath 
in  a  certain  direction,  will  have  all  this 


ELECTROMOTIVE  FORCES.  47 

metallic  coating  gradually  dissolved  if  the 
Current  be  sent  through  the  bath  in  the 
opposite  direction;  for,  in  all  cases  of 
electro -plating,  the  metal  is  only  deposit- 
ed on  one  of  the  conducting  surfaces  con- 
nected with  the  poles;  i.e.,  on  the  nega- 
tive, and  is  dissolved  from  a  plate  of  metal 
connected  with  the  opposite  or  positive 
pole.  Since,  in  an  alternating- current 
circuit,  both  the  article  to  be  plated  and 
the  plating  metal  become  alternately  pos- 
itive and  negative,  it  might  be  supposed 
that  it  would  be  inrpossible  to  produce 
any  permanent  plating  whatever  by  such 
a  current,  and,  although  this  is  true  to  the 
extent  of  preventing  plating  from  being 
carried  out  practically  by  such  methods, 
nevertheless,  permanent  electro -plating 
effects  can  be  produced  by  alternating 
currents,  when  certain  relations  exist 
between  the  size  of  the  article  to  be 


48       ALTERNATING  ELECTRIC  CURRENTS. 

plated  and  the  strength  of  the  current 

passing. 

^ 

So  far  as  the  heating  effects  of  the  elec- 
tric current  are  concerned,  alternating 
currents  produce  the  same  amount  of  heat 
that  continuous  currents  do.  For  ex- 
ample, if  an  incandescent  lamp  be  con- 
nected to  a  continuous -current  circuit  of 
110  volts  pressure,  and,  subsequently,  to 
an  alternating -current  circuit  of  110  volts 
pressure,  the  amount  of  light  and  heat, 
which  the  lamp  will  give  off,  will  be  the 
same  in  both  cases. 

A  marked  difference  exists  between  the 
physiological  effects  of  an  alternating  and 
a  continuous  current.  When  a  continuous 
current  is  sent  through  the  human  body, 
chemical  and  physiological  effects  are  pro- 
duced, entirely  distinct  from  those  which 


ELECTKOMOTIVE    FORCES.  49 

attend  the  passage  of  an  alternating  cur- 
rent under  similar  circumstances.  When 
passing  through  the  vital  organs  of  the 
body,  any  electric  current,  whether  con- 
tinuous or  alternating,  may,  if  sufficiently 
powerful,  cause  death.  Alternating  cur- 
rents, however,  at  commercial  frequencies 
and  pressures,  are  much  more  apt  to  pro- 
duce fatal  effects  on  the  human  body  than 
continuous  currents.  In  New  York  State, 
alternating  electric  currents  are  used  for 
the  execution  of  criminals,  and,  when 
properly  employed,  produce  absolute, 
instantaneous,  and  painless  death.  ^ 

The  experiments  of  Tesla  and  others 
have  shown  that  at  frequencies  and  pres- 
sures far  higher  than  those  employed  for 
ordinary  commercial  purposes,  the  physi- 
ological effects  of  alternating  currents  be- 
come less  severe,  and  that  at  extraordina- 


50        ALTEENATING  ELECTEIC  CUEEENTS. 

rily  high  frequencies,  enormous  pressures 
may  be  handled  with  impunity. 

It  should  be  remembered,  however,  that 
the  physiological  effects  produced  by  a 
current  depend  largely  on  the  resistance 
offered  to  its  passage  through  the  body 
by  the  skin.  For  example,  when  an  alter- 
nating current  is  sent  through  the  human 
body,  by  immersing  the  hands  in  saline 
solutions  connected  with  an  alternating- 
current  circuit,  a  pressure  even  as  low  as 
five  volts  will  usually  produce  very  pain- 
ful sensations.  Care,  therefore,  should  al- 
ways be  taken  in  handling  the  wires 
from  any  high-pressure  electric  source 
particularly  if  that  source  be  one  supply- 
ing alternating  currents. 

In  an  alternating -current  circuit,  both 
the  strength  and  the  direction  of  the  E.  M. 
F.  and  current  are  periodically  varying, 


ELECTKOMOTIVE   FORCES.  51 

being  at  certain  times  at  greatest  strength 
and  at  others  entirely  absent.  It  is  evi- 
dent that  it  would  not  be  correct  to  estimate 
the  value  of  an  E.  M.  F.  or  a  current  at 
either  its  greatest  or  its  least  value ;  nor  is 
it  usual  to  take  the  average  value.  In- 
stead of  this  a  certain  value,  both  of  the 
E.M.F.  and  the  current,  called  respective- 
ly the  effective  E.  M.  F.  and  the  effective 
current  strength, s\xe  taken  as  estimated  from 
their  equivalent  heating  effects.  Thus,  an 
alternating- current  pressure  of  100  volts 
is  one  which,  as  already  mentioned,  will 
produce  in  an  incandescent  lamp  the  same 
heating  and, 'therefore,  the  same  degree  of 
illumination  as  100  volts  of  continuous- 
current  pressure.  In  the  same  way  an  al- 
ternating-electric current,  whose  values  at 
different  successive  instants  in  any  cycle 
would  be  considerably  above  or  below  one 
ampere,  would  be  regarded  as  having  an 


52        ALTERNATING  ELECTRIC  CURRENTS. 

effective  current  strength  of  one  ampere, 
if  it  produced  the  same  heating  effect  in  a 
coil  of  wire  as  a  continuous  electric  cur- 
rent of  one  ampere. 

This  method  of  estimating  the  values  of 
alternating  E.  M.F.'s  and  currents  is  uni- 
versally employed,  and  entirely  dispenses 
with  the  necessity  for  a  determination  of 
the  shapes  of  the  alternating -current 
waves,  just  as  any  method  of  measuring 
tides,  which  depended  upon  a  measure- 
ment of  the  total  quantity  of  water  moved 
up  stream  during  each  tide,  would  dis- 
pense with  the  necessity  for  determining 
the  exact  shape  of  the  tidal  wave. 


CHAPTER  III. 

UNIPHASE  ALTEKNATORS. 

DURING  the  last  few  decades  there  has 
been  witnessed  a  marvelous  development 
in  the  commercial  applications  of  electric- 
ity. Perhaps  the  most  striking  feature  in 
this  development  is  to  be  found  in  the 
strength  of  the  electrical  currents  em- 
ployed to  day,  as  compared  with  the 
strength  of  those  which  were  commercial- 
ly possible  only  a  few  years  ago.  Elec- 
tricity has  commercially  entered  fields, 
which,  but  a  comparatively  short  time  ago, 
.would  have  been  closed  to  it  by  reason  of 
the  expense  attending  its  production. 

This  development  has  not  been  ren- 
dered possible  so  much  by  improvements 


54        ALTEKNATING  ELECTKIC  CUKRENTS. 

in  the  apparatus  operated  by  electricity, 
as  it  has  been  in  the  improved  methods  for 
producing  electricity  more  cheaply.  For 
example,  to  take  the  field  of  electric  light- 
ing, in  which  the  most  marked  develop- 
ments were  first  manifested;  although 
the  arc  lights  of  to-day  are,  in  their  way, 
marvels  of  mechanical  and  electrical  inge- 
nuity, yet,  in  point  of  fact,  they  do  not  dif- 
fer radically,  in  their  general  construction, 
from  those  produced  fifty  years  ago. 
Why  then  did  riot  these  early  arc  lamps 
enter  into  more  general  use  ?  Surely  not 
on  account  of  any  lack  of  appreciation  on 
the  part  of  the  general  public,  of  the  ad- 
vantages possessed  by  the  voltaic  arc  as 
an  artificial  illuminant,  but  because,  in 
those  early  days,  the  only  practical  means 
for  producing  electrical  currents  was  an 
expensive  and  inconvenient  source  of 
electric  supply;  namely,  the  primary,  or 


UNIPHASE   ALTERNATORS.  55 

voltaic  battery.  That  which  rendered 
electric  lighting,  as  well  as  most  of  the 
many  other  commercial  developments  of 
electricity  which  followed  in  its  wake, 
commercially  possible  was  the  production 
of  a  means  for  cheaply  producing  elec- 
tricity, on  a  large  scale;  viz  the  invention 
of  the  generator  known  as  the  dynamo  - 
electric  machine. 

It  is  a  well -recognized  principle,  in  the 
physical  world,  that  in  order  to  perform 
work  of  any  kind,  whether  mechanic- 
al, chemical  or  electrical,  energy  must  be 
expended.  Consequently,  the  production 
of  a  definite  amount  of  electrical  energy 
requires  the  expenditure  of  a  definite 
amount  of  work. 

A  machine  is  a  device  for  transforming 
one  kind  of  work  into  another.  Thus  a 
steam  engine  and  boiler  form  a  machine 


56         ALTEENATING  ELECTRIC  CUEEENTS. 

for  transforming,  into  mechanical  work, 
the  work  of  heat,  liberated  by  the  burn- 
ing of  coal.  Despite  the-  fact  that  the 
steam  engine  has  been  repeatedly  im- 
proved, since  the  early  days  of  Watt,  in 
1765,  yet  in  the  best  forms  of  triple -ex- 
pansion engines,  as  produced  to-day,  the 
work  delivered  by  the  engine  amounts  to 
but  about  sixteen  per  cent,  of  the  work 
delivered  by  the  coal;  so  that,  although 
the  steam  engine  can  transform  the  work 
of  heat  into  mechanical  motion,  it  throws 
away,  during  the  process  of  transforma- 
tion, five  parts  out  of  every  six.  Contrast- 
ing with  this  the  modern  dynamo  machine, 
the  latter  will  be  found  a  far  more  effi- 
cient agent  for  the  transformation  of  ener- 
gy ;  for,  even  in  small  sizes,  of  say  one  H.P. , 
it  is  capable  of  delivering,  as  electrical 
work,  75  per  cent,  or  about  three  parts  out 
of  every  four,  of  the  mechanical  work  ex- 


UNIPHASE  ALTERS  ATO&S.       5? 

pended  in  driving  it,  while  in  large  sizes, 
of,  say  thousands  of  H.P.  it  is  capable  of 
delivering  as  electrical  energy  98  per  cent, 
of  the  mechanical  energy  it  receives. 

Although  in  practice  dynamo -electric 
machines  are  generally  driven  by  steam 
engines,  yet  their  economy  over  other 
electrical  sources  is  so  great  as  to  war- 
rant this  use,  despite  the  low  efficiency 
of  the  steam  engines.  Since  the  expense 
of  maintaining  steam  power  decreases 
markedly  with  the  size  of  the  steam  plant, 
and  since,  as  we  have  seen,  the  capability 
of  the  dynamo  increases  with  its  size, 
it  is  generally  found  more  expedient, 
in  practice,  to  generate  electrical  currents 
in  large  quantities  at  a  few  points  called 
central  stations,  distributing  the  electrical 
power  to  consumers  by  means  of  suitable 
distribution  circuits,  than  it  is  to  have 


58        ALTERNATING  ELECTRIC  CURRENT^. 

as  many  individual  plants  as  there  are 
consumers  of  the  electric  current.  This 
is  especially  the  case  where  dynamos 
are  driven  by  cheap  water  power. 

A  visit  to  any  central  station,  where 
electricity  is  being  generated  on  a  com- 
mercial scale,  will,  on  even  a  casual  obser- 
vation, enable  one  to  readily  divide  the 
machinery  into  two  distinct  classes;  name- 
ly, the  driving  machinery  and  the  driven 
machinery.  The  driving  machinery  will 
consist  either  of  steam  engines  or  of 
water  wheels.  The  driven  machinery 
will  consist  of  various  forms  of  dyna- 
mos. The  driving  and  driven  machinery 
are  connected  together,  either  by  means 
of  belting  or  ropes,  or  are  rigidly  coupled 
together  on  the  same  shaft. 

At  first  sight  it  may  seem  that  different, 
types  of  dynamo  machines  differ  radically 


UNIPHASE  AKTEBtfAtfORS.  59 


in  their  detailed  construction.  A  closer 
inspection,  however,  will  show  that  such 
differences  are  apparent  rather  than  real; 
for  it  will  then  be  seen  that  all  have  cer* 
tain  parts  in  common;  namely,  the  part 
called  the  armature,  in  which  the  electric 
current  is  generated,  and  the  part  called 
the  field  magnet  in  which  the  magnetic 
field  of  the  machine  is  generated. 

Attention  has  already  been  called,  in  the 
second  chapter,  to  the  fact  that  when  loops 
of  wire  are  filled  and  emptied  with  mag- 
netic flux,  electromotive  forces  are  gener- 
ated in  the  wire.  The  dynamo  -electric 
machines  that  we  see  operating  in  a  central 
station,  are  devices  for  filling  and  empty- 
ing, with  magnetic  flux,  conducting  loops 
that  are  placed  on  the  armature  of  the 
machine.  In  order  to  do  this,  either  the 
armature  or  the  field  is  rotated.  Usually 


60        ALTEKNATING  ELECTKIC  CURRENTS. 

it  is  the  armature  that  is  rotated,  since 
the  armature  is  generally  the  lighter  part. 

The  E.  M.  F.  generated  in  such  conduct- 
ing loops,  reverses  its  direction  twice  dur- 
ing each  rotation  of  the  armature  in  a  bi- 
polar field;  i.  e.,  a  field  having  one  north 
and  one  south  pole.  All  dynamo -elec- 
tric machines  are  capable  of  ready  division 
into  two  sharply  marked  classes;  namely, 
those  in  which  alternating  E.  M.  F.'s  are 
delivered  to  the  consumption  circuits,  that 
is,  the  circuits  external  to  the  machine, 
producing  in  them  alternating -electric  cur- 
rents, and  those  in  which  such  E.  M.  F.'  s 
are  commuted,  or  given  the  same  direction, 
by  means  of  devices  called  commutators. 
In  other  words,  all  dynamo -electric  ma- 
chines can  be  divided  into  alternating- 
current  dynamos  or  alternators,  and  con- 
tinuous-current dynamos. 


TOIPHASE    ALTERNATORS.  61 

/We  have,  therefore,  a  general  principle 
py  means  of  which  we  can  determine 
/whether  or  not  a  given  machine,  which 
we  are  examining  in  a  central  station, 
is  an  alternator,  or  a  continuous -current 
dynamo,  since,  in  the  case  of  the  al- 
ternator, the  conducting  loops  of  wire 
on  the  armature  are  connected  direct- 
ly to  the  external  circuit,  generally  by 
means  of  brushes  resting  on  simple  col- 
lector rings,  while  in  continuous -current 
dynamos,  the  brushes,  instead  of  resting 
on  collector  rings,  rest  on  a  commutator, 
which  differs  from  the  rings  in  the  fact  that 
it  consists  of  a  number  of  separate  con- 
ducting bars,  insulated  from  one  another. \ 

The  preceding  principle,  however,  needs 
some  modification,  since  the  require- 
ments of  electrical  engineering,  some- 
times, render  it  advisable  to  construct  dy- 


62        ALTERNATING  ELECTBIC  CTTBBENTS. 

namos  so  as  to  render  them  capable  of  giv- 
ing simultaneously  both  alternating  and 
continuous  currents.  Various  methods 
are  adopted  to  obtain  this  result.  For  ex- 
ample, in  some  cases  a  portion  of  the  con- 
ducting  loops  on  the  armature  have  the 
alternating  E.  M.  F.'s  generated  in  them  so 
commuted  as  to  produce  a  continuous 
current,  while  the  remaining  loops  are 
connected  directly  to  the  collector  rings, 
from  which  the  alternating  currents  are 
carried  off  to  the  consumption  circuits,  by 
means  of  brushes  resting  on  the  rings. 
In  such  cases,  the  continuous  currents 
are  employed  for  various  purposes,  gener- 
ally for  the  excitation  of  the  magnetic  field 
through  which  the  armature  revolves, 
which  excitation  must  always  be  provided 
by  continuous  currents. 

In  all  alternators,  therefore,  continuous 


UNIPHASE   ALTERNATORS.  63 

currents  must  be  provided  to  flow  through 
the  field  coils.  Such  continuous  currents 
are  either  supplied  by  the  machine  itself, 
by  commuting  a  portion  of  the  conducting 
loops  on  the  armature,  or  are  supplied 
from  a  separate  source.  In  other  words, 
all  alternators  can  be  divided  into  two 
sharply  marked  classes;  namely,  those 
that  are  self  excitefL  that  is,  supply  their 
own  field  magnets  with  continuous  cur- 
rents, and,  therefore,  must  be  supplied 
with  a  commutator  in  addition  to  the  col- 
lector rings;  and  those  which  are  separate- 
ly excited,  or  which  derive  the  continuous 
currents  for  the  excitation  of  their  field 
magnet  coils  from  some  external  source. 

Let  us  now  examine  some  of  the  dyna- 
mos that  are  commonly  met  with  in  cen- 
tral stations  in  the  United  States.  Take, 
for  example,  the  dynamo  shown  in  Fig.  15. 


64       ALTERNATING  ELECTRIC  CURRENTS, 


FIG.  15.— BIPOLAR  CONTINUOUS-CURRENT  GENERATOR. 


UNIPHASE   ALTERNATORS. 


65 


This  is  a  bipolar  dynamo;  that  is  to  say, 
its  field  magnets  M,  M,  excited  by  large 
coils  of  wire  as  shown,  produce  two  poles, 
S,  between  which  the  armature  A, 


FlG.    16.— QUADRIPOLAR    CONTINUOUS-CURRENT    GENERATOR. 


is  revolved.  An  inspection  of  this  ma- 
chine will  show  that  it  must  belong  to  the 
continuous -current  type,  since  the  brushes 


66        ALTERNATING  ELECTEIC  CURRENTS. 

rest  on  a  commutator  (7,  composed  of  nu- 
merous insulated  copper  bars. 

Fig.  16  shows  a  type  of  quadripolar  dy- 
namo; or  a  dynamo  whose  field  magnet 
coils,  A,  B,  C,  D,  produce  four  poles  be- 
tween which  the  armature  revolves. 
Here  again  this  machine  evidently  belongs 
to  the  continuous -current  type,  since  its 
brushes,  in  this  case  four  sets,  evidently 
rest  on  a  commutator,  M. 

Fig.  17  shows  a  type  of  separately -excit- 
ed alternator.  Here  a  small  continuous 
current  dynamo  D1 ,  provided  with  a  com- 
mutator at  (7,  supplies  a  continuous  cur- 
rent through  the  brushes  B,  to  the  con- 
ductors 1  and  2,  to  the  12  field  magnets  M, 
M,  etc.,  of  the  alternator  D. 

In  any  bipolar  generator,  whether  con- 


UNIPHASE   ALTEENATOES. 


67 


FIG.  17- — SEPARATELY-EXCITED  ALTERNATOR. 

tinuous  or  alternating,  the  two  poles  are 
respectively  North  and  South.  In  a  quad- 
ripolar  machine,  guch  as  represented  in 


68        ALTERNATING  ELECTRIC  CURRENTS. 

Fig.  16,  the  poles  are  alternately  North 
and  South;  and,  in  general,  in  generators 
containing  any  number  of  poles,  the  polar- 
ity is  alternately  North  and  South,   as 
are  the  12  poles  in  Fig.  17.     A  moment's 
thought  will  show  that  a  multipolar  gen- 
erator must,  therefore,  necessarily  contain 
an  even  number  of  poles,  since  any  odd 
number  of  poles  would  bring  two  poles  of 
the  same  polarity  in  juxtaposition.     In 
the  alternator  shown  in  this  figure,  the 
currents  produced  by  the  armature  are 
carried  to  the  external  circuit,  as  alternat- 
ing currents,  by  means  of  brushes  resting 
on  the  collector  rings  R,  R,  which,   ac- 
cording to  the  principles  already  explained 
in  Chapter  II,  become   alternately  posi- 
tive and  negative  during  the  rotation  of 
the  armature  past  each  pole. 

Fig.  18  shows  another  form  of  separately- 


UNIPHASE   ALTEKNATOKS. 


69 


excited  alternator.  Here  the  continuous  - 
current  generator,  instead  of  being  sepa- 
rate from  the  machine,  and  connected  with 
it  by  a  belt,  as  in  Fig.  1 7,  is  mounted  on  the 


FIG.  18.— SEPARATELY-EXCITED  ALTERNATOR. 

same  shaft  as  the  alternator  at  Dl ,  and  a 
continuous  current,  taken  from  the  com- 
mutator and  brushes  B,  is  led  to  the  field 
magnets  M,  M,  of  the  alternator  D.  The 
alternating  currents  produced  in  this  gen- 


70        ALTERNATING  ELECTKIC  CURRENTS. 

erator  are  carried  to  the  external  circuit 
by  means  of  brushes  resting  on  the  collect- 
or rings  R,  R.  The  main  driving  pulley 
of  the  machine  is  shown  at  P. 

Heretofore,  all  the  generators  we  have 
examined  have  had  but  a  single  circuit  of 
wire  on  their  field  magnet  coils.  Some- 
times, however,  it  is  necessary  to  provide 
two  separate  circuits  in  the  exciting  coils 
on  the  field  magnets.  Such  machines  are 
called  compound- wound,  or  composite  ma- 
chines. The  object  of  double-winding  on  the 
field  magnets  is  to  maintain  automatically 
the  same  pressure  at  the  terminals  or 
brushes  of  the  alternator,  whether  it  is 
supplying  a  strong  or  a  weak  current  in 
its  circuit;  or,  as  it  is  sometimes  termed, 
to  regulate  automatically  the  pressure 
under  all  loads.  Fig.  19  represents  such 
a  self -regulating  compound-wound  alterna- 


UNIPHASE   ALTEKNATORS. 


71 


tor.  Here  one  of  the  circuits  on  the  field 
magnets  M,  M,  is  separately  excited  by  the 
continuous  -  current  generator  Dlf  The 
other  circuit  on  the  field  magnets  is  ex- 


FIG.      19 —  COMPOUND -WOUND,      SEPARATELY  -  EXCITED 
ALTERNATOR. 

cited  by  a  portion  of  the  alternating  cur- 
rent supplied  by  the  machine,  and  which 
is  commuted  by  a  commutator  C,  The 


72        ALTERNATING  ELECTRIC  CURRENTS. 

alternating  current  is  carried  to  the  ex-    • 
ternal  circuit  by  the  rings  R,  R. 

The  electrical  connections  of  such  a 
compound-wound  machine  are  shown  in 
Fig.  20.  Here  the  exciter  D19  sends  from 


FIG.  30.— DIAGRAM  OF  CONNECTIONS  IN  A  PARTICULAR  COM- 
POUND-WOUND, SEPARATELY- EXCITED  ALTERNATOR. 

its  brushes  a  continuous  current  through 
an  adjustable  resistance,  or  regulating  de- 
vice called  a  rheostat,  and  through  a  fine 
wire  circuit  to  the  field  coils  M,  M,  which 
are  connected  in  series.  The  coils  C\  C, 
Cv  etc. ,  mounted  on  the  revolving  armature 
Al  generate  alternating  currents,  which  are 


UNIPHASE   ALTEBNATOftS.  73 

connected  to  the  collecting  rings  R,  R, 
and  to  the  commutator  C,  as  shown; 
namely,  one  end  is  connected  directly  to 
the  collecting  ring  R,  and  the  other  end 
to  the  ring  R  l ,  through  the  commutator 
C.  Under  these  circumstances  a  certain 
portion  of  the  current  passes  around  the 
commutator  through  the  path  marked  G. 
S.  shunt,  of  German  silver  wire,  passing 
on  as  alternating  currents  to  the  collect- 
or ring,  and  by  means  of  the  brushes  to 
the  external  circuit  or  line,  as  alternating 
currents,  while  the  remainder,  or  com- 
muted portion,  is  fed  through  the  brushes 
B,  B,  to  the  coarse  wire  circuit  of  the 
field  magnets.  The  effect  of  this  arrange- 
ment is,  that  as  the  strength  of  the  cur- 
rent supplied  to  the  external  circuit  in- 
creases, the  portion  of  this  current  sup- 
plied to  the  coarse  wire  circuit  of  the  field 
magnets  increases,  and  the  field  magnets 


74       ALTEBNATIHG  ELECTBIC   CURRENTS. 

are  thereby  strengthened,  thus  increasing 
the  E.  M.  F.  of  the  machine  by  increasing 
the  magnetic  flux  passing  through  the 
coils  on  the  armature. 

A  self -excited  alternator  supplies  from 
its  own  armature,  through  a  commu- 
tator, all  the  current  required  for  the  exci- 
tation of  its  field  magnets .  All  alternators 
may,  therefore,  be  divided  into  three  gen- 
eral classes;  namely, 

(1)  Separately -excited  machines,  in  which 
the  currents  required  for  the  excitation  of 
the  field  magnets  are  obtained  from  a  con- 
tinuous-current dynamo.     Such  alterna- 
tors employ  no    commutators    but    only 
a  pair  of  collector  rings. 

(2)  Self -excited  machines,  which  supply 
all  the  current  required  for  the  excitation 
of  their  field  magnets,  after  such  current 
has  been  rendered  continuous  by  the  ac- 


UKIMASE  ALTEBtfAToHS.  75 


tion  of  a  commutator.  Such  machines, 
therefore,  employ  a  commutator  in  addi- 
tion ;bo  collector  rings. 

(3)  Compound-  wound  alternators,  which 
consist  practically  of  a  combination  of  the 
two  preceding  types.  In  other  words,  the 
principal  excitation  of  the  field  magnets  is 
obtained  from  a  separate  dynamo,  while 
the  additional  excitation,  needed  to  main- 
tain a  constant  pressure  at  the  collector 
rings  under  all  conditions  of  load,  is  ob- 
tained from  their  own  armature  current 
through  the  action  of  a  commutator. 

Fig.  21  represents  a  self-excited  alter- 
nator with  a  commutator  at  C,  and  col- 
lector rings  at  R,  R,  for  the  delivery  of 
alternating  currents  to  the  circuit. 

In  order  to  familiarize  the  reader  with 
the  varieties  of  alternators  in  common  use 
in  the  United  States,  two  additional  ex- 


76        ALTERNATING  ELECTBIC  CURRENTS. 

amples    of  alternating -current   machines 
are  given  in  Figs.  22  and  23.     An  examina- 


FIG.  'Jl.— SELF-EXCITING  ALTERNATOR. 

tion  of  these  figures  will  show  that  the  ma- 
chines represented  belong  to  the  same  gen- 
eral type  as  those  already  described,  the 


UNIPHASE   ALTERNATORS. 


77 


differences  being  either  in  mechanical 
construction  or  in  the  relative  arrange- 
ment of  the  parts.  For  example,  Fig.  22 


FIG.  22.— 2000-LiGHT  ALTERNATOR. 

shows  a  separately -excited  alternator  of 
10  poles  with  collector  rings  at  R,  R,  sup- 
plying alternating  currents,  through  the 
leads  1  and  2,  to  the  external  circuit.  The 


78        ALTERNATING  ELECTRIC  CURRENTS. 

separate  exciter  Dl ,  supplies  commuted  or 
continuous  currents  to  one  winding  of  the 
field  magnets,  M,  M,  and  part  of  the  ar- 
mature current  from  the  alternator  D,  is 
supplied  through  the  commutator  (7,  to 
the  other  winding  of  the  field  coils.  This 
machine  is,  therefore,  a  compound- wound, 
separately -excited  alternator,  and  agrees  in 
all  essential  electrical  features  with  the 
machine  shown  in  Fig J9. 

Fig.  23  shows  a  form  of  alternator  in 
which  only  a  pair  of  collector  rings  is 
employed.  Here  the  separate  exciter, 
necessary  for  supplying  continuous  cur- 
rents to  the  field  magnets,  is  not  shown, 
and,  as  there  is  no  commutator  on  the 
machine,  it  is  clearly  not  compound - 
wound.  This  alternator  corresponds  elec- 
trically to  the  type  of  machine  shown 
in  Fig.17. 

Beside  the  forms  of  alternators  above 


UNIPHASE  r ALTERNATORS.  79 

described,  there  are  many  others.  All, 
however,  possess  the  same  fundamental 
features  although  these  features  may  dif- 


FIG.  23.— 1000-LiGHT  ALTERNATOR. 

fer  markedly  in  their  construction  details. 
For  example,  in  some  alternators  the  ar- 
mature is  fixed  and  the  field  rotates.  In 
others,  both  armature  and  field  are  fixed, 


80        ALTERNATING  ELECTKIC  CURRENTS. 

but  a  rotating  frame  is  so  placed  in  rela- 
tion to  both  as  to  generate  E.  M.  F.  's  in 
the  conducting  loops  or  coils  on  the  ar- 
mature. Such  alternators  are  called  in- 
ductor alternators. 


CHAPTER  IV. 

P  O  W  E  11. 

VISITING  an  electric  central  station  at 
the  time  of  full  load,  that  is,  when  the 
station  is  generating  its  full  electric 
power,  it  is  evident  that  a  great  deal  of 
energy  is  being  expended  or  work  being 
done.  The  fires  under  the  boilers  are 
working  at  full  draft;  the  steam  engines 
are  working  at  full  steam  pressure  and 
speed,  and  the  dynamos,  if  belt-driven, 
are  receiving  practically  all  the  energy 
liberated  by  the  engines  through  their 
tightened  connecting  belts.  Evidently, 
therefore,  the  driving  machinery  is  trans- 
mitting an  enormous  amount  of  power  to 
the  driven  machinery.  Indeed,  not  infre- 
quently several  thousand  horse-power  are 


5Z        ALTEKNATING  ELECTRIC  CURRENTS. 

thus  delivered  in  large  central  stations, 
from  the  steam  engines  to  the  dynamos. 
But  there  is  no  immediate  evidence  to  the 
eye,  as  to  what  becomes  of  all  this  power. 
Our  everyday  experience  would  lead  us 
to  expect  some  more  evident  effect  pro- 
duced by  the  expenditure  of  so  much 
power.  Were  the  engines  suitably 
mounted  on  wheels  and  placed  on  a  rail- 
road track,  the  same  amount  of  power  ap- 
plied to  driving  wheels  would  be  sufficient 
to  carry  the  entire  plant  along  the  road  at 
a  considerable  speed.  In  the  central  sta- 
tion, however,  the  energy  is  transformed 
into  electrical  energy  which  is  being  si- 
lently carried  away  by  the  conductors. 
These  silent  conductors,  however,  are 
capable  of  delivering  up  the  energy  given 
to  them  at  various  points  along  their  cir- 
cuit, and  if  all  this  energy  were  employed 
to  drive  electric  motors,  the  total  work 


POWER.  83 

which  could  be  performed  by  such  mo- 
tors, provided  no  loss  occurred  in  trans- 
mission, would  of  course  be  equal  to  that 
developed  by  the  steam  engines. 

It  is  evident,  then,  that  a  circuit  con- 
veying an  electric  current,  may,  in  its 
turn,  be  regarded  as  a  source  of  driving 
power  by  which  the  motors  are  driven. 
But  in  the  case  of  the  steam  engine,  there 
is  an  evident  connection  between  the 
driving  and  the  driven  dynamo ;  namely, 
the  belting  or  shafting.  There  must  also 
be  some  connection  between  the  dynamo 
as  a  driving  and  the  motor  as  the  driven 
machine.  Here  the  connection,  though 
far  less  evident,  consists  in  the  conducting 
circuit  connecting  the  dynamo  and  motor; 
or,  in  other  words,  the  conducting  circuit, 
and  its  electric  activity,  take  the  place  of 
the  driving  belt. 


84       ALTERNATING  ELECTRIC  CURRENTS. 

Suppose  a  water  motor  is  operated  in 
a  city  by  the  flow   of  water  through  a 
pipe,  connected  with  a  reservoir   on  an 
adjoining  hill.     Here,  clearly,  the  source 
of  energy  received  by  the   motor  is  the 
moving  or  falling  water.     This  energy,  in 
its  turn,  was  received  from  a  pump  which 
raised  the  water  into  the  reservoir,  from, 
possibly,  a  river  or  lake  at  a  lower  level. 
Moreover,  the  amount  of  energy  received 
by  the  motor  is  perfectly  definite,  since 
each    pound    of    water,   falling    from    a 
height  of  one  foot,  conveys  an  amount  of 
work  called  a  foot-pound,  so  that,  if  the 
reservoir  contains  a  million  pounds  of  wa- 
ter, and  the  difference  of  level  between 
the  reservoir  and  the  motor  is  100  feet, 
then  the  total  source  of  work  upon  which 
the    motor   can  draw,   is    100x1,000,000 
or  100,000,000  foot-pounds. 

This   stock  of  power  in  the  reservoir 


PO>VEK.  85 

might  be  expended  by  the  water-motor  in 
a  day,  or  in  an  hour,  according  to  the  rate 
at  which  the  motor  works,  and,  therefore, 
permits  the  water  to  flow  from  the  reser- 
voir. In  other  words,  the  ability  of  the 
unreplenished  reservoir  to  keep  the  motor 
running  for  a  given  time,  depends  upon 
what  is  called  the  activity  of  the  motor,  or 
the  rate  at  which  it  is  doing  work.  This 
activity  is  usually  expressed  in.  foot-pounds 
per  second,  or  in  foot  pounds  per  minute. 

The  commercial  unit  of  activity  is  the 
horse-power,  or  550  foot-pounds  per  sec- 
ond. If,  then,  the  motor  be  a  one  horse- 
power motor,  and,  for  simplicity  of  cal- 
culation, be  supposed  to  be  a  perfect  ma- 
chine; i.e.,  to  waste  no  power  in  friction, 
then  the  flow  of  water  through  the  pipe 
will  be  5  1-2  pounds  per  second,  and  this 
quantity  of  water  falling  one  hundred  feet 


86        ALTERNATING  ELECTRIC  CURRENTS. 

in  one  second  will  produce  an  activity 
of  5  1-2  x  100=550  foot-pounds  per  second. 

Although  electricity  is  not  a  liquid  like 
water,  yet,  since  many  of  the  laws  which 
control  the  flow  of  water  are  also  applic- 
able to  the  flow  of  electricity,  it  is  con- 
venient, in  considering  the  manner  in 
which  an  electric  current  is  able  to 
transmit  power  to  a  motor,  to  regard 
electricity  as  though  it  were  a  fluid  in 
actual  motion.  As  in  the  case  of  water 
in  motion,  the  amount  of  activity  trans- 
mitted can  be  expressed  by  the  pounds  of 
water  flowing  per  second,  multiplied  by 
the  difference  of  level  in  feet  through 
which  it  flows,  so  in  the  case  of  an  elec- 
tric current,  the  activity  transmitted  can 
be  expressed  by  the  rate-of-flow  of  elec- 
tricity in  coulombs-per-seconds  multiplied 
by  the  difference  of  electric  pressure 


u 

POWER.  87 

through  which  it  flows,  expressed  in  volts. 
Moreover,  as  the  activity  in  the  current  of 
moving  water  is  expressed  in  foot-pounds 
per  second,  of  which  550  make  a  horse- 
power, so  the  activity  in  the  current  of 
electricity  is  expressed  in  volt- coulombs 
per  second,  or  in  icatts,  of  which  746  make  a 
horse -power.  If,  therefore,  we  multiply 
the  rate  of  electric  flow  in  a  circuit,  ex- 
pressed in  amperes,  by  the  difference  of 
electrical  level  or  pressure,  expressed  in 
volts,  the  product  will  be  the  activity  in 
the  electrical  circuit  expressed  in  watts, 
746  watts  being  equal  to  one  horse-power. 

The  activity,  or  the  rate  of  delivering 
power  from  a  water  reservoir,  can  be  in- 
creased either  by  increasing  the  difference 
of  level,  or  by  increasing  the  rate-of-flow; 
so  in  an  electric  current,  the  activity,  or 
the  rate  of  delivering  electric  power,  can 


88        ALTERNATING  ELECTRIC  CURRENTS. 

be  increased  either  by  increasing  the  dif- 
ference of  electrical  level  in  volts,  or  by  in- 
creasing the  rate  of  electric  flow  in  amperes. 

Steam  engines  are  generally  rated  in 
horse-power  (contracted  H.P.);  that  is  to 
say,  a  one-horse-power  steam  engine  is 
capable  of  doing  an  amount  of  work  equal 
to  550  foot-pounds  per  second.  A  one- 
horse-power  steam  engine,  therefore,  is 
capable  of  lifting  a  pound  weight  550  feet 
high,  or  100  pounds  5  1-2  feet  high,  in 
each  second  of  time. 

Electric  generators  are  usually  rated 
in  watts;  but  since  a  watt  is  so  small 
a  unit  of  activity,  being  only  1 -746th  of  a 
horse-power,  the  kilowatt  or  1000  watts  is 
the  unit  generally  adopted.  Thus,  a  1000- 
watt  generator,  or  a  1  KW.  generator, 
might  supply  one  ampere  in  its  circuit  at 


POWEE.  89 

a  pressure  of  1000  volts,  between  its 
brushes;  or,  it  might  supply  50  amperes 
at  a  pressure  of  twenty  volts,  or  1000  am- 
peres at  a  pressure  of  one  volt. 

The  following  examples  of  electrical 
activities,  required  for  the  operation  of 
apparatus  in  common  use,  may  prove  of 
interest: 

An  ordinary  incandescent  lamp,  of  10- 
candle-power,  requires  about  50  watts,  so 
that  at  this  rate  one  electrical  horse -power 
will  supply  nearly  fifteen  lamps.  The 
pressures  at  which  such  lamps  are  com- 
monly operated  are  either  about  100  volts 
or  50  volts.  A  100-volt  IG-candle-power 
lamp,  will,  therefore,  usually  take  a  cur- 
rent of  approximately  1-2  ampere,  since 
100  volts  x  1-2  ampere  =  50  watts;  while 
if  the  lamp  be  intended  for  a  fifty-volt  cir- 
cuit, it  will  require  a  current  of  one  am- 


90        ALTERNATING  ELECTRIC  CURRENTS, 

pere,  An  incandescent  lamp*  therefore, 
requires  about  l-15th  of  a  H.P,  or  about 
37  foot-pounds  per  second  to  be  supplied 
to  it  at  its  terminals  in  electrical  energy, 

An  arc  lamp,  of  the  ordinary  2000  can- 
dle-power rating,  usually  requires  some 
450  watts  for  the  production  of  the  arc  at 
a  pressure  of  45  volts.  This  represents  a 
current  strength  of  10  amperes,  since  45 
volts  x  10  amperes  =  450  watts.  An  arc 
lamp,  therefore,  requires  to  be  supplied 
with  an  activity  of  about  3-5ths  of  an  elec- 
trical horse -power;  or,  in  other  words,  for 
every  arc  lamp  supplied  to  a  circuit,  the 
engine  driving  the  arc  light  generator 
must  supply  3-5ths  of  a  horse-power,  and 
something  over  for  losses  in  transmission. 

An  electric  current  of  5000  amperes, 
supplied  from  a  central  station  to  a  net- 


work  of  trolley  conductors,  in  a  street 
railway  system,  under  a  pressure  of  550 
volts  at  the  dynamo  brushes,  will  repre- 
sent a  total  activity  of  550x5000=2,750,- 
000  watts,  or  2750  KW.  or  3686  H.  P.  in 
electrical  energy  supplied  to  conductors. 

Although,  as  we  have  seen,  the  rate  of 
work  or  activity,  in  a  continuous  current, 
is  equal  to  the  number  of  amperes  multi- 
plied by  the  number  of  volts,  yet  when 
we  come  to  apply  the  same  rule  to 
the  case  of  alternating  currents,  we  find 
that  it  is  only  true  under  certain  circum- 
stances. 

This  is  for  the  reason  that,  in  the  con- 
tinuous-current circuit,  the  pressure  is 
al\\  ays  acting  to  drive  the  current  in  the 
direction  in  which  it  is  already  moving, 
while  in  an  alternating -cur  rent  circuit  it 
may  be  at  times  aiding  the  current  for- 


92        ALTERNATING  ELECTRIC  CURRENTS. 


ward,  and  at  times  opposing  it.  Fig.  24 
represents  the  current  strength  in  an  al- 
ternating-current circuit,  and  also  the  E. 
M.  F.  of  the  generator  by  which  that  cur- 


b.0 
O       01 


FIG.  24.— WAVES  OF  ALTERNATING  E.  M.  F.  AND  CURRENT 
IN  STEP. 


SI 


m 


\ 


FIG.  25.— WAVES  OF  ALTERNATING  E.  M.  F.  AND  CURRENT  IN 
A  CIRCUIT,  OUT  OF  STEP. 


poWEit.  93 

rent  strength  is  produced.  The  two  sets 
of  waves  are  seen  to  be  in  step,  the  crests 
of  the  E.  M.  F.  waves  coinciding  with 
the  crest  of  the  current  waves.  In  such 
a  case  the  product  of  the  effective  volts 
and  the  effective  amperes  gives  the  elec- 
tric activity,  just  as  in  the  case  of  a  con- 
tinuous-current circuit.  Fig.  25,  however, 
represents  the  more  usual  case  in  whiclk 
the  pressure  or  E.  M.  F.  is  in  advance  of 
the  current.  It  will  be  observed  that  at 
the  moment  when  the  pressure  has  its 
greatest  value,  or  rises  to  the  crest  of  its 
wave,  the  current  strength  will  not  have 
reached  the  crest  of  its  wave,  the  result 
will  be  that  the  pressure  will  have  dropped 
below  the  zero  line  00,  or  will  have  become 
negative,  while  the  current  is  still  above 
the  zero  line,  or  in  the  positive  direction. 
In  other  words,  the  pressure  or  E.  M.  F., 
instead  of  aiding  the  current  at  this  in- 


94        ALTERNATING  ELECTRIC  CURRENTS. 

stant,  is  opposing  it.  Under  these  circum- 
stances if  we  multiply  the  effective,  num- 
ber of  volts  by  the  effective  number  of  am- 
peres, we  shall  obtain  an  activity  which 
is  greater  than  that  actually  produced  in 
the  circuit.  In  other  words,  the  appar- 
ent activity  in  watts  will  be  greater  than 
the  actual  activity  in  watts,  and  the  dis- 
crepancy will  depend  upon  the  distances 
between  the  crests  of  the  pressure  and 
current  waves;  i.  e.9  upon  the  amount 
of  time,  in  each  period,  during  which  the 
pressure  is  opposing,  instead  of  driving. 
|  The  apparent  activity,  has,  therefore,  to 
;  be  multiplied  by  a  quantity  called  the 
power  factor,  in  order  to  obtain  the  real 
activity.  The  value  of  the  power  factor 
depends  upon  the  difference  of  phase. 
The  waves  of  current  and  pressure  are 
said  to  be  in  phase,  or  in  step,  when  their 
crests  and  troughs  occur  simultaneously ; 


95 


and  when  the  waves  of  pressure  become 
separated  from  the  waves  of  current,  the 
two  waves  are  said  to  differ  in  phase. 

Even  in  an  alternating  -current  circuit, 
under  certain  circumstances,  if  we  take 
for  both  of  these  quantities  their  effective 
values,  as  we  have  heretofore  pointed  out, 
the  activity  is  correctly  represented  by 
the  product  of  the  E.  M.  F.  by  the  current. 
This  would  be  the  case  in  a  circuit  of  in- 
candescent lamps  where  the  circuit  is  prac- 
tically free  from  loops,  since,  in  such  a  cir- 
cuit, induction  is  practically  absent.  Such 
a  circuit  is  sometimes  called  an  induction- 
less  circuit;  but  when,  as  is  the  case  in  most 
practical  alternating  -current  circuits,  con- 
ducting loops,  in  the  shape  of  coils  of 
wire,  are  present,  then,  as  we  have  already 
pointed  out,  the  successive  filling  and 
emptying  of  these  loops  with  magnetic  flux, 


96       ALTERNATING  ELECTKIC  CUEKENTS. 

on  the  rapid  periodical  increase  and  de- 
crease in  current  strength,  will  set  up  E. 
M.  F.'s  in  the  wire,  counter  or  opposed 
to  the  E.  M.F.'s  producing  such  flux,  so 
that  the  combined  effect  of  the  impressed 
and  the  counter  E.  M.  F.'s  produces  what 
is  called  a  resultant^.  M.  F.  which  is  shift- 
ed in  position,  or  differs  in  amount  and 
phase  from  the  impressed  E.  M.  F.  But 
with  this  resultant  E.  M.  F.  the  current  is 
always  in  step.  This  resultant  E.  M.  F., 
multiplied  by  the  current  in  step  with  it, 
gives  the  true  activity  of  the  current. 
Since,  however,  the  circumstances  pro- 
ducing the  displacement  of  the  current,  in 
phase,  are  often  complex,  it  is  well  to  multi- 
ply the  impressed  E.  M.  F.  by  the  current 
and  introduce  a  power  factor  rather  than 
to  determine  what  the  resultant  E.  M.  F. 
in  the  circuit  may  be.  For  example,  an  in- 
candescent lamp,  supplied  direct  from 


POWER."  97 

mains  at  an  alternating  pressure  of  100 
volts,  may  take,  say  half  an  ampere  of  cur- 
rent. The  activity  in  the  lamp  will  be 
100x1-2=50  watts,  and  the  power  factor 
is  («ne,  or  100  per  cent.  This  is  for  the 
reason  that  there  is  no  reactance  in  the 
lamp,  and  the  current  waves  through  its 
filament  are  almost  exactly  in  step  with 
the  waves  of  pressure  at  its  terminals. 
Consequently,  the  activity  of  the  lamp, 
and  the  light  it  emits,  will  be  the  same, 
whether  it.be  connected  to  100  volts  alter- 
nating or  continuous  pressure. 

If,  however,  the  same  alternating- cur- 
rent mains  be  connected  with  a  coil  of 
many  turns,  the  resistance  of  which  is 
the  same  as  that  of  the  lamp  filament, 
while  the  continuous  current  will  be  half 
an  ampere  as  before,  the  alternating  cur- 
rent will  be  much  less,  perhaps,  only  l-10th 


98        ALTERNATING  ELECTRIC  CURRENTS. 

of  an  ampere,  this  being  due  to  the 
reactance  of  the  coil,  as  already  explained. 
The  activity  in  the  continuous  current 
will  be  50  watts,  but  in  the  alternating  cur- 
rent it  will  not  be  100  x  l-10th  or  10  watts, 
but  considerably  less,  because  the  waves 
of  pressure  and  current,  owing  to  the  re- 
actance of  the  coil,  are  out  of  phase,  and 
the  power  factor  of  the  coil  will  be  less 
than  say  30  percent.,  making  an  elec- 
trical activity  in  the  coil  only  10x30- 
100ths=:3  watts. 


CHAPTER  V. 

TRANSFORMERS. 

IF  we  leave  the  central  station  and  fol- 
low an  alternating- current  circuit,  erected 
upon  poles,  up  to  the  first  point  where 
the  current  is  utilized,  we  will  probably 
see  apparatus  of  the  general  type  repre- 
sented in  Fig.  26,  either  placed  upon  a 
pole,  as  shown  in  the  figure,  or  in  some 
convenient  location  on  the  side  of  a  build- 
ing. Such  an  apparatus  is  called  a  trans- 
former, and  is  only  employed  on  alternat- 
ing-current circuits.  It  remains  now  to 
examine  the  general  construction  of  alter- 
nating-current transformers,  and  the  part 
they  take  in  the  economical  distribution 
of  electric  currents  over  extended  areas. 

If  an  alternator,  at  a  central  station,  is 


100     ALTERNATING  ELECTRIC  CURRENTS. 

supplying  100  volts  at  its  collector  rings, 
a  100 -volt  lamp  connected  at  the  brushes 
of  such  a  machine  will  burn  at  full  incandes  - 


FIG.  26.— ALTERNATING-CURRENT  TRANSFORMER  WITH  DIRECT 
SERVICE  WIRES. 

cence  or  brilliancy.  Suppose,  now,  that  this 
alternator  be  connected  to  a  pair  of  wires 
five  miles  in  length.  If  the  lamps  were 


TRANSFORMERS. 


101 


connected  to  the  lines  as  shown  in  Fig.  27, 
at  distances  of  1,  2,  3,  4  and  5  miles  re- 
spectively, we  should  find  that  the  bril- 
liancy of  the  lamps  diminished  as  the  dis- 
tance from  the  alternator  increased;  the 
reason  being  that  the  pressure,  or  voltage, 
between  the  lines  at  the  lamp  terminals, 
would  decrease  as  we  receded  from  the 


(3 


FIG.  27.— DIAGRAM  ILLUSTRATING  THE  FALL  OF  ELECTRIC 
PRESSURE  OR  VOLTAGE  ALONG  A  CIRCUIT. 

alternator.  This  decrease  in  pressure  of 
electricity  flowing  from  an  alternator, 
through  a  long  conductor,  finds  its  ana- 
logue in  the  decrease  of  the  pressure  of 
water  flowing  from  a  reservoir  through  a 
long  pipe  as  shown  in  Fig.  28.  If  the 
reservoir  supply  water  through  a  pipe, 
and  pressure  gauges  be  connected  at  dif- 
ferent distances,  say  1,  2,  3,  4  and  5  miles, 


102    ALTERNATING  ELECTRIC  CURRENTS. 

as  shown,  then,  when  the  flow  is  entirely 
shut  off  at  the  distant  end,  assuming  no 
leakage  through  the  pipe,  the  gauges  will 
all  show  the  same  pressure;  but  when  the 
flow  is  fully  established  through  the  pipe, 
the  gauge  at  the  outflow,  where  the  wa- 


El  sH  5 


FIG.  28.— DIAGRAM  ILLUSTRATING  THE  FALL  OF  HYDRAULIC 
PRESSURE  ALONG  AN  OUTFLOW  PIPE. 

ter  escapes,  will,  owing  to  the  loss  of  head, 
or  drop  of  pressure,  arising  from  the  fric- 
ti^^^^^wat^J^jffie^^^gijS^  the 
lowest  pressure.  The  pressure  at  inter- 
mediate distances  between  the  reservoir 
and  the  outflow,  will  be  intermediate  be- 
tween the  pressure  at  the  reservoir  and 


TRANSFORMERS.  103 

the  pressure  at  the  outflow.  Similarly, 
in  an  electric  circuit,  the  resistance 
offered  by  the  conductors  to  an  electric 
flow  produces  a  drop  of  pressure,  so  that 
under  the  conditions  shown,  the  most  dis- 
tant lamps  will  only  receive  say  70  volts, 
while  the  intermediate  lamps  will  receive 
pressures  intermediate  between  110  and 
70  volts.  The  fall  of  pressure  depends  on 
the  size  of  the  wire  and  the  strength  of 
the  current  required  for  each  lamp. 

An  ordinary  incandescent  lamp  of  16- 
candle -power  requires  to  be  supplied,  as 
already  stated,  with  an  activity  of  about 
50  watts.  Since,  in  the  preceding  case, 
the  pressure  is  assumed  to  be  100  volts, 
each  lamp  would  take  approximately  1-2 
ampere  of  current  (100  volts  x  l-2ampere  = 
50  watts).  If  the  lamp  could  be  construct- 
ed so  that  it  would  properly  operate  when 


104     ALTERNATING  ELECTRIC  CURRENTS. 

supplied  with  say  l-20th  of  an  ampere,  or 
10  times  less  current,  the  current  supplied 
by  an  alternator  to  such  lamps,  under  simi- 
lar conditions,  would  be  10  times  less,  and 
the  drop  of  pressure  in  the  mains  would, 
therefore,  be  10  times  less,  since  the  drop 
of  pressure  in  any  conductor,  expressed 
in  volts,  is  always  equal  to  the  current 
which  it  carries  in  amperes  multiplied  by 
its  resistance  in  ohms.  But  such  a  50- 
watt  lamp,  taking  only  l-20th  ampere, 
would  have  to  be  designed  for  a  pressure 
of  1000  volts  (1000  volts  x  l-20th  ampere 
=  50  watts).  Such  lamps  can  not  be  con- 
veniently made  at  the  present  time,  and 
even  if  they  could  be  made,  1000  volts  is 
an  unsafe  alternating- electric  pressure  to 
introduce  into  a  building.  The  only  way 
in  which  this  troublesome  drop  of  pressure 
can  be  avoided,  without  the  use  of  special 
apparatus,  when  the  best  arrangement  of 


TRANSFOKMEKS.  105 

wires  has  been  adopted  for  the  distribu- 
tion of  light,  is  to  decrease  one  of  the  fac- 
tors on  which  the  value  of  the  drop  de- 
pends; namely,  to  decrease  the  resistance 
of  the  wires,  by  increasing  their  size  and 
weight.  In  other  words,  we  can  always 
decrease  the  drop  indefinitely,  by  increas- 
ing the  size  of  the  conductors  indefinitely. 
But  heavy  conductors  of  copper  are  ex- 
pensive, and  a  point  is  soon  reached 
when  the  distance,  to  which  electric 
supply  can  be  carried  from  a  central  sta- 
tion to  lamps,  is  commercially  impossible. 

Happily,  however,  the  use  of  trans- 
formers with  alternating  currents  renders 
it  possible  to  obtain  all  the  advantages  of 
high-pressure  transmission  and  yet  read- 
ily to  reduce  such  pressure  to  50  or  100 
volts  within  the  building  it  is  desired  to 
supply.  The  corresponding  conditions  of 


106     ALTERNATING  ELECTRIC  CURRENTS. 

hydraulic  transmission  are  represented 
in  Fig.  29  where  a  long  pipe,  PP,  of  small 
cross -section,  carries  water  from  a  reser- 
voir R,  at  a  high  pressure  and  enters  the 


FIG.  29. — DIAGRAM  REPRESENTING  LONG  DISTANCE  WATER 
POWER  TRANSMISSION  THROUGH  SMALL  PIPE  AT  HIGH 
PRESSURE,  WITH  TRANSFORMATION  TO  LARGE  PIPE,  Low 
PRESSURE,  LOCAL  SYSTEM. 

high-pressure  cylinder  of  a  pump  M,  con- 
nected with  a  large,  low-pressure  cylinder 
of  the  pump  M,  which  drives  forward  a 
large  quantity  of  water  from  a  local  reser- 


TKANSFOEMEBS.  107 

voir  at  a  reduced  pressure,  through  a  large 
pipe  p  p,  to  the  water  motor  in  its  vicinity. 
By  such  an  arrangement,  therefore,  it  is 
possible  to  transmit  water  power  to  a  great 
distance  by  a  small  pipe,  and  yet  deliver 
a  large  volume  of  water  to  a  motor  which 
is  designed  to  be  operated  at  a  low  pres- 
sure. In  the  same  way,  by  the  use  of 
alternating  currents  in  connection  with 
transformers,  it  is  possible  to  obtain  all 
the  advantages  to  be  derived  from  the 
transmission  of  high  pressure  electric  cur- 
rents over  small  wires,  and  yet  so  trans- 
form or  change  the  pressure  at  the  point 
of  consumption  as  to  permit  the  use  of 
incandescent  lamps  that  will  only  oper- 
ate economically  under  low  pressures. 

It  has  already  been  pointed  out  that  the 
value  of  the  electrical  activity  transmitted 
by  any  circuit  when  the  power  factor  is 


108    ALTEBNATItfG  ELECTBIC  CUKKENTS. 

100  per  cent,  or  unity,  is  equal  to  the  prod- 
uct of  the  amperes  multiplied  by  the 
volts,  and  it  is  clear  that  a  small  electric 
current,  carried  at  a  high  pressure,  say  10 
amperes  at  1000  volts,  would  give  the  same 
amount  of  activity,  namely,  10  KW.,  as 
would  a  current  of  100  amperes  at  100  volts, 
but  would  require  a  much  smaller  wire. 

An  alternating -current  transformer  is  a 
device  for  enabling  electric  energy  to  be 
economically  transmitted  at  high  pressure 
and  low  current  strength,  to  the  point  of 
delivery,  and  then  reducing  or  transform- 
ing this  supply  to  a  large  current  at  a  cor- 
respondingly lower  pressure. 

Let  us  inquire  into  the  means  where- 
by a  transformer  is  capable  of  performing 
this  important  function.  To  do  this  we 
will  first  examine  its  construction.  The 


TRANSFORMED. 


109 


alternating- current  transformer  consists 
essentially  of  two  coils  of  wire,  one  usually 
coarse  and  the  other  fine,  the  fine  wire 
coil  being  of  much  greater  length  and  hav- 
ing a  greater  number  of  turns  than  the 
coarse  wire.  Fig.  30  shows  one  of  the  sim- 


FIG.    30.  —  SIMPLE      FORM      OF       ALTERNATING- CURRENT 

TRANSFORMER. 

t ' 

plest  forms  of  transformers.  It  consists, 
as  shown,  of  two  coils  of  wire  P  and  S, 
wound  on  a  core  of  iron  wire.  When  an 
alternating  current  is  sent  through  coil  P, 
called  the  primary  coil,  it  will,  by  induc- 
tion, produce  an  alternating  E.  M.  F.,  of 
the  same  frequency,  in  the  coil  S,  which 


110    ALTERNATING  ELECTRIC  CURRENTS. 

is  called  the  secondary  coil,  and  this  second- 
ary E.  M.  F.  is  employed  to  send  an  alter- 
nating current  through  the  lamps  or  other 
apparatus  which  are  to  be  operated.  In 
the  case  supposed,  the  high-pressure  cur- 
rent would  be  sent  through  the-  primary 
coil  P,  whose  terminals  are  connected  to 
the  line,  and  the  low-pressure  current 
would  be  induced  in  the  secondary  coil  S, 
whose  terminals  are  connected  as  shown 
with  the  apparatus  to  be  operated. 

The  alternating-  current  transformer 
operates  as  follows:  On  the  passage,  of 
the  alternating  current  through  the  pri- 
mary coil  P,  the  coil  become  alternately 
magnetized  in  opposite  directions;  that  is 
to  say,  its  loops  become  successively 
filled  and  emptied  with  an  oscillating 
magnetic  flux.  The  coil  thereby  has  a 
counter  E.  M.  F.  set  up  in  it,  or,  in  other 


TRANSFORMERS.  Ill 

words,  acts  as  a  choking  coil.  At  the 
same  time,  the  flux  through  the  iron  core 
successively  fills  and  empties  the  second- 
ary coil  S,  and  thereby  induces  in  it  an 
E.  M.  F.  which  will  alternate  at  the  same 
frequency  as  that  in  the  primary.  If  the 
circuit  of  the  secondary  coil  is  open;  i.  e., 
disconnected  from  its  apparatus,  the  pres- 
ence of  this  secondary  E.  M.  F.  will  not 
affect  the  reactance  or  choking  effect  of 
the  primary  coil;  but  if,  on  the  contrary, 
the  secondary  circuit  be  closed  through 
its  load  of  lamps,  motors,  or  other  appa- 
ratus, the  current  in  the  secondary  coil 
will  tend  to  magnetize  the  core  in  the  op- 
posite direction  to  that  of  the  primary 
coil,  and  so  diminish  the  reactance  of  the 
primary  winding.  The  choking  effect  of 
the  primary  coil  will,  thereby,  be  reduced 
as  the  secondary  current  and  the  load  in- 
crease. In  other  words,  the  transformer 


112    ALTERNATING  ELECTRIC 

becomes  self -regulating,  the  choking  ef- 
fects of  the  primary  coil  automatically 
varying  so  as  to  permit  the  right  amount 
of  current  and  power  to  be  received  from 
the  high  pressure  mains,  in  order  ade- 
quately to  supply  the  secondary  or  low 
pressure  consumption  circuit. 

Let  us  now  examine  the  pressures 
which  exist  in  the  primary  and  secondary 
circuits.  If  each  coil  P  and  S9  has  the 
same  number  of  turns,  the  E.  M..  F.  in- 
duced in  the  secondary  will  be  practical- 
ly the  same  as  that  supplied  or  impressed 
upon  the  terminals  of  the  primary,  so  that 
there  would  be  no  transformation  or 
change  as  regards  pressure  and  current. 
If,  however,  the  secondary  coil  is  made  up 
of  but  half  the  number  of  turns  in  the 
primary  coil,  the  flux  passing  through  the 
iron  core  only  links  with  half  the  number 


TKANSFORMEKS.  113 

of  secondary  turns  that  it  links  with  in 
the  primary  coil,  and  the  E.  M.  F.  in- 
duced in  the  secondary  will  be  but  half 
.as  great  as  that  in  the  primary.  If  the 
primary  impressed  E.  M.  F.  were  1000 
volts  effective,  that  in  the  secondary  cir- 
cuit would  be  about  500  volts.  Again, 
if  the  secondary  coil  contain  say  one 
tenth  of  the  number  of  turns  existing  in 
the  primary  coil,  then  its  E.  M.  F.  would 
be  correspondingly  reduced  and  would 
become  approximately  100  volts.  If  in 
this  case  the  wire  forming  the  secondary 
coil  were  maintained  of  the  same  diameter 
as  that  in  the  primary  coil,  the  small  sec- 
ondary coil  would,  for  the  same  electrical 
activity  in  each  circuit,  have  to  carry  ten 
times  the  current  strength  which  is  sup- 
plied to  the  primary.  It  would  be  neces- 
sary, therefore,  to  increase  the  cross- 
section  of  the  secondary  coil,  say  ten 


114    ALTERNATING  ELECTRIC  CURRENTS. 

times,  so  that  the  bulk  of  the  two  will,  in 
practice,  be  approximately  the  same. 

It  is  evident  that  if  the  coil  S,  assumed 
in  the  last  condition  to  contain  one  tenth 
of  the  number  of  turns  in  the  coil  P,  could 
be  connected  to  the  high-pressure  termi- 
nals, or,  in  other  words,  be  employed  as 
the  primary,  that  the  coil  P,  would  have 
an  E.  M.  F.  induced  in  it,  whose  value 
would  be  ten  times  as  great  as  that  in  the 
mains.  Transformers  may,  therefore,  be 
divided  into  two  sharply-marked  classes; 
namely,  step-down  transformers,  where  the 
pressure  in  the  secondary  is  less  than  the 
primary  pressure,  and  step -up  transform- 
ers, where  the  pressure  in  the  secondary 
is  greater  than  the  primary  pressure.  In 
actual  practice,  transformers  are  not  built 
in  the  exact  manner  shown  in  the  last 
figure.  The  primary  and  secondary  coils 


TEANSFOEMEES.  115 

may  be  variously  disposed  as  regards  each 
other,  but  in  all  cases  they  are  brought 
as  close  together  as  possible,  and  are 
so  surrounded  by  laminated  iron  as  to 
cause  the  flux  produced  by  the  primary 
to  pass  through  or  become  linked  with  all 
the  turns  of  the  secondary.  Since  the 
coils  may  assume  various  positions,  it  is 
evident  that  different  types  of  transform- 
ers may  differ  radically  in  their  appear- 
ance. They  will,  however,  all  possess 
the  same  essential  features;  namely,  pri- 
mary and  secondary  coils,  and  a  laminated 
iron  core  common  to  both. 

Fig.  31  represents  a  laminated  iron  core 
C,  of  sheet  iron  stampings,  having  a  form 
resembling  that  shown  in  Fig.  32,  within 
the  hollow  spaces  of  which  are  inserted  the 
two  coils  P  and  S9  one  being  the  primary 
coil  of  say  1000  turns  of  fine  wire,  and  the 


116  ALTERNATING  ELE  CTKIC  CURRENTS. 

other  the  secondary  coil  (for  convenience 
divided  into  halves)  with  a  total  of  say  100 
turns  of  coarser  wire.  Since  the  primary 
coil  may  be  connected  to  mains  at  say 
1000  volts  pressure,  and  is  in  close  juxta- 
position to  the  secondaiT  coil,  from  which 
wires  are  carried  into  the  building  to  be 


FIG.  31. — TRANSFORMER  SHOWING  INTERNAL  CONSTRUCTION. 

supplied  by  the  current,  it  is  evident  that 
the  insulation  of  the  two  coils  from  each 
other  must  be  carefully  preserved,  since, 
otherwise,  the  pressure  of  1000  volts  might 
be  led  into  the  building.  In  order  to  en- 
sure a  high  degree  of  insulation,  the  coils 
are  sometimes  immersed  in  an  insulating 


TRANSFORMERS.  117 

oil.  TJie  transformer  coils  shown  in  Fig. 
31  at  A,  are  placed  in  the  iron  vessel 
shown  at  B,  which  is  then  filled  with  oil. 

Another  form  of  oil-insulated,  step-down 
transformer  is  shown  in  Fig.  33.     Here  the 


FIG.  32.— SHEET  IRON  STAMPING. 
For  Transformer  Shown  in  Fig.  31. 

primary  coil  has  its  ends  brought  out  at 
p,  p,  and  its  secondaries  at  s,  s,  divided,  as 
before,  into  two  halves  for  convenience. 
This  transformer  is  enclosed  as  shown  in 
Fig.  34,  in  a  box  filled  with  oil,  the  pri- 


118      ALTERNATING  ELECTRIC  CURRENTS. 

mary  terminals  being  brought  out  through 
the  fuse-box  at  P  and  P,  and  the  second- 
ary terminals  at  S  and  S. 

In  order  to  prevent  the  current  gener- 


FIG.  33.— 100-LiGHT  TRANSFORMER  WITHOUT  Box. 

ated  in  the  secondary  circuit  from  becom- 
ing dangerously  great,  should  an  accident- 
al short-circuit  occur  in  the  wires  of  the 


TRANSFORMERS. 


119 


building  supplied,  a  device  called  &  fuse- 
block  is  employed  with  transformers. 
This  device  consists  of  an  iron  box  con- 
taining lead  fuse  wires  which  are  inserted 


FIG.  34.— 100-LiGHT  STANDARD  TRANSFORMER. 

+ 

in  the  primary  circuit,  so  that  the  cur- 
rent from  the  high-pressure  mains,  in  or- 
der to  reach  the  primary  coil,  has  to  pass 
through  these  fuse  wires.  The  fuse  wires 


120    ALTERNATING  ELECTKIC  CURRENTS. 

are  composed  of  a  lead  alloy  of  such 
size  that  they  carry  safely  the  normal 
working  current  of  the  transformer,  but, 
on  an  undue  excess  of  current,  become  so 
heated  as  to  melt,  and  open  the  circuit, 
thus  automatically  disconnecting  the 
transformer  from  the  mains.  The  porce- 


FIG.  35. — FUSE-BOX  AND  FUSES. 

lain  or  earthenware  fuse -block  is  shown 
in  Fig.  35  at  B,  with  a  fuse  wire  WW, 
laid  across  it,  having  its  ends  clamped 
under  connection  screws.  The  box  is 


TRANSFORMERS. 


121 


provided  with  the  lid  L,  so  that  when  the 
fuses  have  been  melted  or  ''blown'"  new 
wires  can  be  readily  inserted. 

Another  form  of  transformer  fuse-box  is 


FIG.  36.— DETAILS  OP  TRANSFORMER  FUSE-BOX. 

shown  in  Fig.  36,  detached  from  its  trans- 
former case.  Here,  two  unglazed  porce- 
lain handles  H,  H,  are  inserted  by  hand 
into  two  separate  porcelain  apartments  in 
an  iron  box.  Within  these  compartments 


122     ALTERNATING  ELECTRIC  CURRENTS. 

are  brass  contact  pieces,  only  one  of  which 
S19  is  visible  in  the  figure,  so  arranged 
that  when  the  handle  H9  H,  is  pressed 
home  into  the  compartment,  connection  is 
maintained  between  them  through  the 
fuse  wires,  W9  W9  clamped  between  bind- 
ing posts  T,  T,  and  connected  with  flexible 
plugs  S9  S9  which  fit  into  the  receptacles 
Sl .  The  lid  L,  is  provided  for  closing  the 
box.  The  advantage  of  this  particular 
form  is  that  when  the  handles  are  pushed 
in,  thus  connecting  the  transformer  with 
the  high-pressure  mains  P,  P,  the  sudden 
or  explosive  fusing  of  the  wire  cannot 
injure  the  operator,  whose  hand  is  pro- 
tected by  the  back  of  the  handle  H. 

Fuse  wires  are  also  inserted  in  the  sec- 
ondary circuits  of  the  transformer,  some- 
times in  the  transformer  itself  as  at  S9  in 
Fig.  26,  and,  sometimes,  in  separate  fuse- 
boxes  within  the  building. 


123 

We  have  seen  in  Figs.  31,  33  and  34, 
that  the  secondary  coils  are  divided  into 
two  separate  halves.  The  advantage  of 
this  method  lies  in  the  fact  that  some 
houses  have  their  lamp  circuits  wired  for 
50  volts  pressure,  and  others  for  100  volts 
pressure.  If  now,  the  coils  of  each  of 
the  two  separate  circuits  of  such  a  trans- 
former, having  a  pressure  of  50  volts,  are 
so  arranged  that  the  current  passes  from 
one  secondary  coil  through  the  next  in 
succession,  so  that  the  two  coils  are  con- 
nected as  though  they  formed  an  unbro- 
ken winding,  then  their  E.  M.  F.  's  will  be 
added,  making  a  total  of  100  volts.  On 
the  contrary,  if  it  be  .desired  to  use  a 
pressure  of  but  50  volts,  then  the  two 
coils  are  employed  side  by  side,  or  so  con- 
nected to  the  house  wires  that  each  of  the 
coils  supplies  half  the  current  delivered. 
Such  connections  are  shown  in  Fig.  37. 


124     ALTERNATING  ELECTRIC  CURRENTS. 

At  A,  100  volts  are  obtained  for  the  sec- 
ondary circuit  by  connecting  the  two  coils 
in  series,  as  it  is  called,  so  that  the  ar- 
rows represent  the  direction  of  the  cur- 
rent at  some  particular  instant.  At  J5, 


FIG.  37.— METHOD  OF  CHANGING  SECONDARY  CONNECTIONS. 

50  volts  are  obtained  by  the  parallel  con- 
nection of  two  coils,  or,  as  it  is  sometimes 
called,  by  their  connection  in  multiple.  If 
at  A,  the  transformer  is  delivering  10  am- 
peres at  100  volts  pressure,  or  1000  watts, 
at  B,  it  will  be  delivering  10  amperes  in 


TRANSFORMERS. 


125 


each  coil,  or  20  amperes  in  all,  at  50  volts 
pressure,  and,  therefore,  also  1000  watts* 

In  Fig.  31.  the  capacity  of  the  transform* 


FIG.  38. — OUTDOOR  TYPE  OF  TRANSFORMER. 

er  represented  is  600  watts,  or  such  as  is 
capable  of  furnishing  current  for  opera- 
ting 12  fifty-watt  lamps.  That  in  Figs.  33 
and  34,  5000  watts,  or  100  such  lamps. 


126      ALTERNATING  ELECTRIC  CURRENTS. 

A  still  larger  transformer  of  7500  watts 
capacity,  or  capable  of  operating  150  fifty- 
watt  lamps,  is  represented  in  Fig.  38. 
This  particular  transformer  is  not  insu- 
lated with  oil,  but  depends  upon  the  in- 
sulating covering  of  its  coils  for  protec- 
tion, the  free  space  within  the  cover  or 
iron  shield  being  filled  with  air. 

The  current  required  to  supply  a  trans- 
former at  full  load  may  readily  be  ascer- 
tained when  the  primary  pressure  is 
known.  For  example,  in  the  case  of  a 
7500 -watt  transformer,  if  the  primary  pres- 
sure is  1000  volts,  the  primary  current 
must  be  7  1-2  amperes  (1000  volts  x  7  1-2 
amperes  =  7500  watts),  if  we  assume  that 
the  primary  power  factor  is  100  per  cent, 
and  that  no  loss  occurs  in  the  transform- 
er. Strictly  speaking,  the  power  factor, 
even  at  full  load,  is  not  quite  100  per 


TRANSFORMERS.  127 

cent.,  and  a  little  loss  of  energy  occurs  in 
the  transformer;  i.e.,  the  transformer  be- 
comes warm  in  doing  its  work,  so  that  the 
current  strength  supplied  from  the  primary 
circuit  at  full  load  must  be  somewhat  in 
excess  of  7  1-2  amperes. 


PIG.  39. — 500-LiGHT  TRANSFORMER,  INDOOR  TYPE. 

A  form  of  25,000-watt  transformer  (25 
KW.  or  about  33  H.  P. )  intended  for  500 
fifty-watt,  16-candle-power  incandescent 
lamps,  is  represented  in  Fig.  39.  This 
transformer  is  intended  to  be  located  in  a 
cellar,  or  other  suitable  place  within  doors. 


128    ALTERNATING  ELECTRIC  CURRENTS. 

It  will  be  seen  that  as  the  capacity  of 
the  transformer  increases ;  i.  e. ,  as  the 
transformer  has  to  supply  more  and  more 
power,  its  dimensions  increase,  but  not 
in  the  same  proportion  as  the  increase  in 
capacity;  so  that  if  a  1  KW.  or  20-light 
transformer  weighs,  in  its  case  complete, 
140  pounds,  or  gives  7  watts  per  pound  of 
total  weight,  a  25  KW.  transformer  will, 
probably,  weigh  only  2000  pounds,  or 
give  12  1-2  watts  per  pound,  while  a  200 
KW.  transformer  will,  perhaps,  give  25 
watts  per  pound,  and  a  1200  KW.  trans- 
former 100  watts  per  pound.  It  is  much 
cheaper,  per  kilowatt  of  output,  to  con- 
struct transformers  in  large  sizes. 

When  a  step -down  transformer  is  em- 
ployed to  reduce  the  pressure  in  a  build- 
ing from  1000  to  100  volts,  it  is  clear  that 
at  the  central  station  supplying  the  mains 


TRANSFORMERS.  129 

leading  to  the  building  a  generator  must 
be  employed  of  1000  volts  E.  M.  F.  or  more. 
This  is  commonly  the  case,  and  alterna- 
ting-current generators  in  the  U.  S.  gen- 
erally produce  either  about  1000  or  2000 
volts  effective  at  their  terminals.  When, 
however,  the  current  has  to  be  trans- 
mitted over  lines  of  great  length,  and  it 
is  necessary,  for  purposes  of  economy  in 
conductors,  to  employ  much  higher  pres- 
sures, say  10,000  volts,  it  is  desirable, 
both  on  the  score  of  safety  and  economy, 
to  employ  a  step-up  transformer,  supplied 
by  the  generator  at  a  lower  pressure, 
rather  than  to  endeavor  to  construct  a 
generator  to  directly  develop  such  pres- 
sure. In  such  cases,  of  course,  these 
step -up  transformers  would  be  connected 
directly  to  the  alternator  terminals. 

Large  step -down  transformers,  intended 


130      ALTERNATING  ELECTRIC  CURRENTS. 

to  supply  an  extended  system  of  mains, 
are  frequently  installed  in  a  small  sub-sta- 
tion, for  which  reason  they  are  sometimes 


FIG.  40.— SUB-STATION  TRANSFORMER. 

called  sub -station  transformers.  Since  such 
transformers  may  take  the  entire  load  of 
a  large  alternator,  they  necessarily  re- 


TKANSFOEMEKS.  131 

quire  to  be  of  considerable  dimensions.  A 
form  of  such  transformer  is  shown  in  Fig. 
40,  its  length  being  about  6  feet.  In  de- 
signing such  transformers  care  is  taken  to 
provide  for  the  dissipation  of  the  heat 
generated  in  their  iron  core  and  conduct- 
ors when  in  action.  Here  the  laminated 
core,  consisting  of  large,  thin  sheets  of 
iron,  forming  the  frame  or  body  of  the  ap- 
paratus, CC, is  closely  linked  with  the  coils, 
c,  c,  c,  c.  The  whole  apparatus  is  carefully 
ventilated  to  permit  of  the  free  access  of 
air  and  the  insulation  of  the  coils  carefully 
preserved  by  means  of  sheets  of  mica. 

Another  advantage  secured  by  the  use 
of  a  few  large  transformers  in  place  of  a 
number  of  smaller  ones  is  a  greater  effi- 
ciency. A  large  transformer  in  the  course 
of  its  daily  duty  will  probably  supply,  to 
its  secondary  circuit,  96  per  cent,  of  the 
energy  it  receives  at  its  primary  terminal, 


132      ALTERNATING  ELECTRIC  CURRENTS. 

only  4  per  cent,  being  lost  in  the  trans- 
formers. The  same  amount  of  power 
being  distributed  by  a  number  of  small 
transformers  might  perhaps  result  on 
the  average  to  a  delivery  of  80  per  cent, 
and  a  loss  of  20  per  cent.  In  other  words 
a  small  transformer  wastes  proportionate- 
ly more  energy  than  a  large  one. 


CHAPTER  VI. 

ELECTKIC   LAMPS. 

HAVING  examined  in  the  previous  chap- 
ters the  method  of  generating  alternating 
currents,  the  means  employed  for  their 
distribution,  and  the  apparatus  by  which 
their  strength  can  be  varied,  it  remains 
to  discuss  some  of  the  different  types  of 
electric  apparatus  to  which  such  currents 
are  supplied.  These  are  of  a  variety  of 
forms,  but  the  most  important,  at  the 
present  time,  are  lamps  and  motors. 

When  ah  electric  current  is  sent  through 
a  conductor  of  high-resistance  and  small 
cross-section,  so  that  a  considerable 
amount  of  electric  energy  is  expended  in 
a  smal]  mass  of  material,  the  conductor  is 


134      ALTERNATING  ELECTRIC  CURRENTS. 

heated,  perhaps,  to  the  temperature  of 
luminosity,  when  it  will  emit  light  and 
heat.  This  is  the  principle  on  which  an 
incandescent  electric  lamp  is  operated;  a 
short  thin  filament  or  thread  of  carbon 
forming  the  high-resistance  conductor. 

The  carbon  filament  acquires  its  high 
temperature  in  a  fraction  of  a  second  after 
the  current  has  been  sent  through  it,  as 
can  be  determined  by  observing  the  time 
which  elapses  from  the  closing  of  the  cir- 
cuit by  turning  the  key  or  switch  of  an 
incandescent  lamp  until  the  lamp  gains 
its  full  incandescence.  In  the  same  man- 
ner,  on  the  interruption  of  the  current  by 
the  opening  of  the  circuit,  an  equally  short 
time  is  required  for  the  lamp  to  lose  its 
brilliancy  especially  in  slender  filaments. 

In  an  alternating -current  circuit,  the 
current  not  only  changes  its  strength  but 


ELECTRIC  LAMPS.  135 

also  changes  its  direction,  during  the  dif- 
erent  parts  of  an  alternation.  Conse- 
quently, twice  in  each  cycle,  at  the  mo- 
ment when  the  change  of  direction  occurs, 
there  can  be  no  current  in  the  circuit, 
as  will  be  evident  from  an  inspection  of 
Fig.  6.  When  alternating  currents  are 
supplied  to  incandescent  lamps  at  a  fre- 
quency of  100  cycles  per  second,  it  is  evi- 
dent that  200  times  in  each  second  there  is 
no  current  passing  through  the  lamp.  It 
might,  therefore,  be  supposed  that  the 
lamp  would  go  out  and  be  relighted  200 
times  a  second.  In  reality  an  incandes- 
cent lamp  tends  to  do  this,  and  would  do 
it  were  it  not  for  the  fact  that  the  intervals 
of  cessation  of  current  are  so  brief  that 
the  lamp  has  not  sufficient  time  in  which 
to  appreciably  cool  down,  so  that  such 
changes  of  temperature  as  do*  occur,  are 
not  visible  to  the  eye,  and  the  lamp  does 


136      ALTERNATING  ELECTRIC  CURRENTS. 

not  visibly  flicker.  In  order,  however,  to 
obtain  this  absence  of  flickering  a  certain 
frequency  of  alternation  is  necessary;  for, 
it  is  evident  that  if  the  frequency  becomes 
very  low,  sufficient  time  will  elapse,  be- 
tween the  current  waves,  to  permit  the 
carbon  to  sensibly  decrease  in  brightness, 
thus  permitting  the  retina  of  the  eye  to 
retain  the  impression  of  flickering.  It 
has  been  found,  in  practice,  that  flickering 
in  an  incandescent  lamp  does  not  occur 
when  the  frequency  of  the  alternation 
exceeds  30  to  35  cycles  per  second.  In 
practice,  in  the  United  States,  alternators 
for  incandescent  lighting  are  usually  de- 
signed to  produce  a  frequency  much  high- 
er, say  from  125  to  135  cycles  per  second. 

When  the  energy  from  an  electric  cur- 
rent is  utilized  in  an  incandescent  lamp, 
by  far  the  greater  part  is  uselessly  ex- 


ELECTKIC   LAMPS.  137 

pended  in  producing  heat,  or  non-luminous 
radiation.  It  has  been  found  that  a  com- 
paratively slight  increase  in  temperature 
will  cause  a  marked  increase  in  the 
amount  of  light  emitted  by  a  glowing 
filament.  Consequently,  the  commercial 
efficiency  of  a  lamp  that  is  its  ability  to 
convert  electrical  energy  into  light  en- 
ergy,will  be  greatly  increased,  by  any  cir- 
cumstance which  will  safely  permit  of  an 
increase  of  temperature  of  its  filament. 
This  can  readily  be  shown  by  applying  suc- 
cessively increasing  pressure  or  voltage  to 
the  terminals  of  a  lamp,  and  so  causing 
greater  current  to  flow  through  it,  the  in- 
crease in  the  current  being  followed  by  a 
marked  increase  in  the  amount  of  light 
given  off.  Were  it  possible  to  double  the 
ordinary  working  temperature  of  the  fila- 
ment of  an  incandescent  lamp,  without 
destroying  it,  we  would  very  markedly  in- 


138      ALTERNATING  ELECTRIC  CURRENTS. 

cease  its  light-giving  power.  In  point  of 
fact  even  a  slight  increase  above  the  or- 
dinary temperature  produces  a  great  in- 
crease in  the  brilliancy  of  the  lamp. 

But  while  an  improvement  is  thus  ob- 
tained in  the  light-giving  power  of  a  lamp, 
the  life  of  the  lamp,  or  the  number  of 
hours  during  which  it  will  continue  to 
give  out  this  light,  is  greatly  diminished. 
The  problem  for  increasing  the  efficiency 
of  an  incandescent  lamp  has,  therefore, 
been  to  obtain  a  conducting  substance 
which  would  continuously  stand  a  high 
temperature.  Carbon  is  the  only  sub- 
stance which  has,  thus  far,  been  found 
available  for  commercial  use.  There  is  a 
certain  temperature  at  which  it  is  found 
most  economical  to  operate  carbon  fila- 
ments, both  in  regard  to  their  amount  of 
light  and  duration  of  life.  Below  this  tern- 


ELECTRIC  LAMPS.  139 

perature,  while  the  life  greatly  increases, 
the  candle-power  rapidly  falls  off.  An  in- 
candescent lamp,  burning  at  dull  red  tem- 
perature, will  have  an  indefinitely  long  life- 
time, while  a  similar  lamp,  operated  at  the 
ordinary  temperatures  commercially  em- 
ployed, will  burn  from  600  to  1800  hours. 

i 

Since  the  filaments  of  incandescent 
lamps  are  made  of  various  lengths  and 
cross -sections,  or,  in  other  words,  since 
their  filaments  have  varying  electrical  re- 
sistances, the  pressures  required  to  pro- 
duce in  them  the  requisite  temperature 
will  necessarily  vary.  In  practice,  lamps 
are  constructed  which  require  pressures 
varying  from  2  volts  to  250  volts.  High- 
pressure  lamps,  of  any  given  candle-power, 
have  long,  thin  filaments,  while  low-pres- 
sure lamps,  of  the  same  candle-power, 
have  short  thick  filaments. 


140      ALTERNATING  ELECTRIC  CURRENTS. 

Various  forms  are  given  to  incandescent 
lamps,  but  all  consist  essentially  of  the 
same  parts;  namely,  an  incandescing  fila- 
ment of  carbon  placed  in  an  exhausted 
glass  chamber  and  connected  with  the  cir- 


FIG.  "1.— 16-C.P.  INCANDESCENT  LAMP. 

cuit  by  a  socket,  the  wires  leading  the  cur- 
rent into  the  lamp  being  automatically 
connected  with  the  circuit  by  the  act  of 
inserting  the  lamp  in  its  socket. 

Some  forms  of  incandescent  lamps  are 
shown  in  Figs.  41, 42,  43  and 44.  Fig.  41  is 
a  form  of  16 -candle -power  lamp  in  exten- 
sive use,  consisting  of  a  filament  bent  in  a 


ELECTRIC   LAMPS. 


141 


single  loop.     The   lamp  base  is  provided 
with  a  screw  thread  for  insertion  in  the 


FIG.  43.— INCANDESCENT  LAMPS  AND  SOCKET. 


142      ALTERNATING  ELECTRIC  CURRENTS. 

screw  socket.     Fig.  42  represents  another 
form  of  lamp,  in  which  the  screw  thread  is 


FIG.  43.— INCANDESCENT  LAMPS. 

in  the  interior  of  the  base,   instead  of 
on   the    external    surface.     This    figure 


ELECTKIC  LAMPS. 


143 


also  shows  a  lamp  inserted  in  the  socket 
which  is  provided  with  a  key  K.  Figs.  43 
and  44  show  another  form  of  incandescent 


FIG.  44.— INCANDESCENT  LAMPS. 


144      ALTERNATING  ELECTEIC  CURRENTS. 

lamp  furnished  with  different  bases ;  a  is 
intended  to  give  10  candle-power,  b,  16 
candle-power,  c,  20  and  dt  32. 

All  the  incandescent  lamps  here  shown 
are  equally  applicable  for  use  on  contin- 
uous or  alternating- current  circuits.  In 
practice,  where  the  area  of  distribution  to 
consumers  is  not  great,  the  continuous 
current  is  usually  employed,  but  where 
the  area  of  distribution  is  large,  and  the 
lighting  scattered,  it  is  usually  more 
economical  to  use  aj^rna_ting  currents  in 
connection  with  step -down  transformers. 

Incandescent  .lamps  as  supplied  from 
step-down  transformers  are  always  con- 
nected in  parallel,  that  is,  the  lamp's  termi- 
nals are  connected  across  the  mains  as 
shown  in  Fig.  45,  which  represents  a  two- 
wire  system  of  distribution.  Sometimes, 


ELECTKIC   LAMPS. 


145 


however,  the  lamps  are  connected  as 
shown  in  Fig.  46,  where  the  20  lamps 
shown  are  connected  between  three  wires 
of  the  three-wire  system  of  distribution 
represented. 


x 


FIG.  45.— Two- WIRE  SYSTEM  OF  MULTIPLE  CONNECTED  LAMPS. 

In  cases,  however,  where  incandescent 
lamps  are  required  for  street  lighting  over 
an  extended  area,  where  the  lights  are, 
therefore,  scattered,  the  systems  of  dis- 


146      ALTERNATING  ELECTRIC  CURRENTS. 

tribution  shown  in  Figs.  45  and  46,  are  too 
expensive,  and  it  is  also  too  expensive 
to  employ  a  special  or  separate  trans- 
former for  each  lamp  post.  In  this 
case  the  method  of  distribution  sometimes 


FIG.  46.— THREE-WIRE  SYSTEM   OF   MULTIPLE    CONNECTED 
LAMPS. 

employed  is  that  represented  in  Fig.  47, 
where  the  lamps  are  connected  in  series, 
the  current  passing  successively  through 


ELECTEIC   LAMPS.  147 

each  lamp.  In  the  method  of  distribu- 
tion shown  in  Figs.  45  and  46,  the  failure 
of  any  one  of  the  lamps  to  operate,  as,  for 
example,  by  the  breaking  of  its  filament, 


v 


FIG.  47.— SERIES  DISTRIBUTION  OF    INCANDESCENT    LAMPS 
WITH  ALTERNATING  CURRENTS. 

does  not  affect  the  supply  of  current  to 
the  other  lamps.  When,  however,  the 
lamps  are  connected  in  series,  the  discon- 
tinuity of  one  lamp  would  open  the  entire 
circuit  were  it  not  for  the  small  choking 


148     ALTERNATING  ELECTEIC  CUEEENTS. 

coil  which  is  placed  as  a  shunt  or  by-path 
to  each  lamp.  While  the  circuit  is  main- 
tained through  the  lamps,  very  little  cur- 
rent passes  through  the  choking  coil,  so 
that  the  waste  of  current  and  energy 
through  the  latter  is  very  small.  If,  how- 
ever, the  lamp  breaks  its  circuit,  the 
choking  coil  carries  the  current  without 
appreciably  affecting  the  supply  to  the 
rest  of  the  lamps  in  the  circuit.  These 
choking  coils  are  represented  in  Fig.  47  as 
being  connected  around  the  terminals  of 
each  lamp.  Fig.  48  represents  such  a 
street  lamp  with  its  choking  coil.  Here 
the  lamp  is  provided  with  an  external 
shade  and  globe  to  protect  it  from  the 
weather.  Fig.  49  gives  a  more  complete 
view  of  the  choking  coil. 

Alternating  currents  are  also  employed 
for  arc  lighting.     As  in  the  case  of  incan- 


ELECTRIC  LAMPS.  149 

descent  lighting,  in  order  to  prevent  the 
variations  in  the  current  strength  from 
producing  marked  flickering  in  the  light, 
a  certain  frequency  is  necessary.  It  has 
been  found  in  practice  that  the  arc  lamps 


FIG.  48.— COMBINED  FIXTURE  AND  REACTIVE  COIL. 

will  show  no  disagreeable  flickering  if  the 
frequency  exceeds  45  cycles  per  second. 

Alternating -current  arc  lamps  do  not 
differ  in  general  construction  from  continu- 
ous-current arc  lamps,  save  in  the  details 


150     ALTERNATING  ELECTKIC   CURRENTS. 

of  their    regulating    mechanism.     Since, 
however,  the  upper  and  lower  carbons  be- 


FIG.  49.— STREET  LAMP  REACTIVE  COIL. 

come  alternately  positive  and  negative, 
the  rate  of  consumption  of  each  carbon  is 
sensibly  the  same.  A  form  of  arc  lamp, 


ELECTRIC   LAMPS. 


151 


FIG.  50.— ALTER- 
NATING-CURRENT 
ARC  LAMP. 


152   ALTERNATING  ELECTEIC  CURRENTS. 

suitable  for  use  for  an  alternating-incan- 
descent circuit  of  either  50  or  100  volts 
pressure,  is  shown  in  Fig.  50.  In  circuit 


FIG.  51.— REACTIVE  COIL  OR  COMPENSATOR  FOR  ARC  LAMPS 
ON  ALTERNATING-CURRENT  CIRCUIT. 

with  the  lamp  or  lamps  is  connected  a 
choking  coil  or  compensator,  as  shown  in 
Fig.  49,  whose  object  is  to  regulate  au- 
tomatically the  amount  of  current  passing 
through  the  lamp. 


CHAPTER 

ELECTRIC   MOTORS. 

IT  is  a  well-known  fact  that  when  a  con- 
tinuous electric  current  passes  through  a 
continuous -current  generator  at  rest,  the 
generator  will  be  set  in  motion.  The  ear- 
ly history  of  this  discovery  still  remains 
in  some  doubt.  It  is  claimed  that  the 
first  observation  of  this  power  of  a  dynamo 
to  act  as  a  motor,  or,  in  other  words,  this 
reversibility  of  the  dynamo,  was  the  result 
of  an  accident,  which  occurred  during  the 
Vienna  Exhibition  of  1873,  when  the  cur- 
rent of  one  generator  was  accidentally  led 
through  the  circuit  of  a  second  generator. 
According  to,  perhaps,  more  credible  ac- 
counts, this  property  was  the  direct  re- 
sult of  research  in  1867.  However  this 


154         ALTERNATING  ELECTRIC  CURRENTS. 

may  be,  the  first  dynamo  that  was  ever 
publicly  exhibited  running  as  a  motor, 
from  the  current  supplied  by  a  similar 
dynamo,  was  at  the  opening  of  the  1873 
Vienna  Exhibition. 

It  is  a  well-recognized  scientific  princi- 
ple that  work  is  never  lost  or.  in  other 
words,  that  the  total  amount  of  energy  ex- 
isting in  the  universe  is  constant.  Work 
may  be  made  to  assume  different  forms, 
but  can  never  be  annihilated.  When,  for 
example,  mechanical  work  is  expended  in 
driving  a  dynamo,  apart  from  certain  ex- 
penditures, all  this  work  is  transformed  in- 
to electrical  work.  When  this  electrical 
work  is  properly  applied  to  the  armature 
of  another  generator  standing  at  rest,  the 
electrical  work  is  transformed  into  me- 
chanical work,  as  is  evidenced  by  the  abil- 
ity of  the  motor  to  drive  machinery.  We 


ELECTBIC  MOTORS.  155 

have  seen  that  a  horse -power  is  equal  to 
an  activity  of  746  watts.  Consequently, 
if  the  electric  motor  were  a  perfect  ma- 
chine; i.  e.,  wasted  no  power,  it  would 
take  746  watts,  from  the  circuit  supply- 
ing it,  for  every  horse -power  it  exerted  in 
its  work;  and,  if  operated  at  a  pressure  of 
100  volts  at  the  mains,  would,  therefore, 
receive  746-100=7.46  amperes,  per  horse- 
power delivered.  Owing  to  the  necessary 
losses  of  energy  in  the  motor,  a  greater 
current  strength  than  this  will  in  practice 
be  needed,  perhaps,  10  amperes,  depend- 
ing, however,  upon  the  size  of  the  motors. 
Large  electric  motors  frequently  possess 
a  very  high  efficiency;  i.  e.9  their  output 
in  mechanical  work  is  very  nearly  equal  to 
their  intake  in  electrical  work.  Since,  as 
we  have  seen,  a  motor  can  readily  be 
driven  at  a  long  distance  from  the  genera- 
tor supplying  it,  is  evident  that  the  elec- 


156    ALTEENATINO  ELECTEIC   CUEEENTS. 

trical  transmission  of  power  possesses 
marked  advantages.  An  example  of  a 
continuous -current  motor  is  shown  at 
Fig.  52. 


PlG.  52.—  CONTINUOUS-CUKKJSJNT  STATIONARY  MOTOR 

If  two  continuous -current  generators, 
similar  in  all  respects,  be  electrically  con- 
nected by  a  circuit  say  one  mile  in 
length,  one  being  driven  by  a  steam  en- 
gine as  a  generator,  while  the  other  is 


ELECTKIC  MOTORS. 


157 


running  at  the  same  speed  as  a  motor, 
then,  as  we  have  already  seen,  the  current 
is  alternating  in  the  armature  of  each  ma- 
chine, but,  owing  to  the  action  of  the 
commutator,  is  continuous  in  the  line 
between  them.  Assuming  the  two  ma- 
chines to  be  running  at  the  same  speed, 
if  the  commutators  are  suddenly  removed 
from  each,  the  two  machines  will  continue 
running,  though  the  current  on  the  line, 
as  well  as  the  current  through  the  arma- 
tures, will  now  be  alternating.  The  two 
machines,  which  must  now  be  regarded 
as  alternating- current  machines,  will  still 
be  acting  as  generator  and  motor,  or  a 
the  driving  and  the  driven  machine. 

In  this  respect,  therefore,  continuous 
and  alternating -current  dynamos  are 
alike;  since,  in  either  case,  one  acting  as 
the  generator  can  drive  the  other  as  the 


158      ALTERNATING  ELECTRIC  CtJRRENTS* 

motor.  They  differ,  however,  in  this  re* 
spect,  that,  whereas,  in  the  case  of  the 
continuous  ^current  circuit,  the  motor  will 
start  from  a  state  of  rest,  and  can  be  driv- 
en either  at  the  same  speed  as  the  gener- 
ator or  at  different  speeds;  in  the  case  of 
the  alternating -current  circuit,  the  motor 
will  not  start  from  a  state  of  rest  and  can 
not  be  operated  until  it  has  been  brought 
up  to  the  same  speed  as  the  generator;  or, 
as  it  is  usually  termed,  until  it  has  been 
brought  into  step  with  it.  Once  the  motor 
has  been  brought  up  to  the  speed  of  the 
generator,  it  can,  if  well  designed,  be 
made  to  take  its  full  load  mechanically 
and  electrically,  without  falling  out  of 
step.  Since  such  an  alternating- current 
motor  will  not  operate  unless  it  is  run- 
ning at  the  same  speed  as  the  driving 
alternator,  it  is  called  a  synchronous 
motor. 


ELECTRIC  MOTORS. 


159 


When  synchronous  motors  are  em- 
ployed, it  is,  therefore,  necessary  to  de- 
vise some  means  whereby  they  can  be 
brought  up  to  their  normal  speed  before 
they  are  connected  with  the  circuit  sup- 


FIQ.  53.—  250-H.P.  ALTERNATING  -  CURRENT  SYNCHRONOUS 
MOTOR 

plying  them.  Various  devices  have  been 
proposed  for  this  purpose.  The  one  in 
most  general  use  is  that  shown  in  Fig.  53. 
Here  the  synchronous  alternating- current 


160     ALTEBNATING  ELECTBIC  CUEBENTS. 

motor  S,  of  250  H.  P.,  is  intended  to 
drive  machinery  by  the  pulley  P,  through 
the  clutch  C.  In  order  to  start  the  motor, 
the  clutch  is  opened,  and  a  small  motor  M, 
called  a  diphase  motor,  which  will  be  de- 
scribed in  a  subsequent  chapter,  is  operat- 
ed, and  drives  the  large  motor  armature 
through  the  friction  pulleys  Q  and  R.  As 
soon  as  the  armature  A,  has,  in  this  way, 
been  brought  up  to  speed,  the  small  mo- 
tor M,  is  disconnected,  and  the  armature 
A,  is  connected  with  its  circuit,  when  it 
takes  alternating  currents,  and  is  ready  to 
receive  its  load  as  a  synchronous  motor. 
The  clutch  C9  is  then  thrown  in,  rigidly 
connecting  the  motor  shaft  with  the  driv- 
ing pulley  P.  Finally,  the  small  driving 
motor  M,  is  moved  back  by  the  handle  H, 
so  that  its  pulley  Q,  is  out  of  contact 
with  the  pulley  R.  The  armature  A,  re- 
ceives its  current  through  the  contact 


ELECTKIC   MOTORS. 


161 


rings  6r,  6r,  at  the  end  of  its  shaft,  and,  by 
means  of  the  commutator  K,  s  applies  the 
continuous  currents  required  for  the  ex- 
citation of  its  own  field  magnets,  in  the 
same  manner  as  though  it  were  a  self- 
excited  generator. 


FIG  54.— ALTERNATOR  WITH  SYNCHRONOUS  MOTORS. 

Fig.  54  represents  a  3000 -volt  alterna- 
tor, suppling  two  synchronous  motors  di- 
rectly from  the  same  pair  of  mains,  the 
starting  motors  'not  being  shown.  The 
pressure  at  the  brushes  of  these  motors  is 
marked  as  being  3000  volts  effective, 


162     ALTERNATING  ELECTRIC  CURRENTS. 


representing  about  4200  volts  at  the  peak 
of  each  alternation  of  pressure. 


II 


WtMHUff 


I    I    I     I    I    I 


I    I 


11 


I 


FIG.  55.— ALTERNATOR  WITH   TRANSFORMER  AND  ITS  SEC- 
ONDARY  CIRCUIT. 

Fig.  55  represents  an  alternator  A,  sup- 
plying a  pair  of  high-pressure  mains  M,  M, 
and  a  primary  coil  P,  of  a  transformer  T, 


ELECTKIC   MOTOKS.  163 

whose  secondary  coil  is  connected  to  the 
arc  lamp  L,  incandescent  lamps  /,  /,  and  a 
synchronous  motor  SM,  all  operated  in 
parallel. 

It  is  evident  that  since  a  synchronous 
motor  has  only  one  speed  of  rotation  and 
requires  some  appreciable  time  to  start 
from  rest  by  auxiliary  means,  that  it  is 
unsuited  to  machinery  which  requires  to 
be  operated  at  varying  speeds  and  for 
intermittent  periods.  For  all  purposes, 
however,  where  the  power  is  required 
continuously,  or  for  many  hours  a  day  at 
a  steady  rate,  as,  for  example,  in  pumping 
or  driving  large  counter  shafts  in  a  ma- 
chine shop,  the  synchronous  motor  is  a 
very  useful  machine, 

Up  to  the  present  time  no  single-phase 
alternating- current  motor,  of  say  more 


164      ALTEENATING  ELECTKIC  CUKKENTS. 


FIG.    56.— ONE-EIGHTH    H.P.    ALTERNATING- CURRENT    FAN 
MOTOR. 


ELECTRIC  MOTORS.  165 

than  half  a  horse-power  in  capacity,  has  yet 
been  produced  in  the  United  States,  which 
is  capable  of  starting  at  fall  load,  from 


FIG.  57  —ALTERNATING -CURRENT  FAN  MOTOR. 

rest,  on  ordinary  alternating  circuits,  and 
which  will  run  with  a  reasonable  amount 
of  economy.  There  are,  however,  a  num- 


166     ALTERNATING   ELECTRIC   CURRENTS. 

ber  of  small  alternating- current  motors, 
some  of  which  operate  with  the  aid  of  a 
commutator,  as,  for  example,  the  fan  mo- 
tor, shown  in  Fig.  56.  Here  the  current 
through  the  fields  is  reversed  at  every  al- 
ternation of  the  alternating  current,  but 
by  means  of  the  commutator,  the  effect  of 
this  reversal  of  magnetism  is  reversed 
upon  the  armature  current,  and  a  contin- 
uous magnetic  pull  produced.  Unfortu- 
nately the  efficiency  of  such  machines  is 
comparatively  small,  so  that  they  are  only 
capable  of  being  employed  in  small  sizes, 
where  economy  is  not  of  much  impor- 
tance. Another  form  of  alternating- cur- 
rent motor  of  this  type  is  seen  in  the 
fan  motor  shown  in  Fig,  57. 


CHAPTER  VIII. 

MULTIPHASED  CURRENTS. 

THE  difficulty  pointed  out  in  the  last 
chapter,  as  regards  the  starting  of  syn- 
chronous motors,  has  led  to  a  special  de- 
velopment in  alternating-current  appara- 
tus called  multiphase  apparatus.  The 
synchronous  motor  is  supplied  by  a  single 
alternating  current.  The  multiphase  mo- 
tor is  supplied  by  more  than  a  single  cur- 
rent. In  practice  either  two  or  three  cur- 
rents are  employed  for  driving  multiphase 
motors,  thus  giving  rise  to  diphase  motors, 
which  are  supplied  by  two  separate  alter- 
nating currents,  and  triphase  motors,  which 
are  supplied  by  three  separate  currents. 
Multiphase  motors,  therefore,  require  spe- 
cial generators  for  the  production  of  the 


168     ALTERNATING  ELECTEIC   CURRENTS. 


currents  they  employ.  We  shall  now  pro- 
ceed to  discuss  the  construction  and  op- 
eration of  diphase  and  triphase  generators. 

It  must  first  be  remarked  that  in  a  di- 


FIG.  58 — RELATION"  BETWEEN  Two  DIPHASE  ALTERNATING 
CURRENTS. 

phase  motor,  for  example,  it  is  not  suffi- 
cient to  simply  supply  to  the  motor  any 
two,  separate,  alternating  currents.  The 


MULTIPHASED  CURRENTS.  169 

proper  operation  of  the  motor  requires 
that  the  two  separate  currents  shall  pos- 
sess a  certain  relationship  to  each  other; 
namely,  that  one  shall  be  a  quarter  of  a 
cycle  in  advance  of  the  other,  as  shown  in 
Fig.  58.  A  diphase  generator,  therefore, 
must  be  constructed  not  only  so  as  to 
produce  two  equal  separate  alternating 
currents,  but  these  alternating  currents 
must  also  have  a  quarter  of  a  cycle  of 
phase  difference  between  them.  Such  a 
condition  will  enable  the  motor  to  start, 
as  well  as  to  preserve  a  uniform  pull  or 
torque  upon  its  driving  shaft. 

A  diphase  motor  is  driven  by  two  sepa- 
rate series  of  electrical  impulses  one  quar- 
ter cycle  apart.  This  condition  finds  an 
analogue  in  the  ordinary  steam  locomo- 
tive, which,  as  is  well  known,  is  driven  by 
two  separate  -steam  cylinders  placed  on 


170    ALTERNATING  ELECTRIC   CURRENTS. 

opposite  sides  of  the  driving  engine.  In 
the  early  history  of  the  steam  locomotive, 
when  but  a  single  cylinder  was  used,  it 
was  found,  at  times,  that  the  engine  could 
not  be  started  from  a  state  of  rest,  since 
it  had  stopped  on  a  dead  centre,  and  re- 
quired, like  the  synchronous  motor,  to 
be  started  before  it  could  by  driven. 
This  difficulty,  as  is  well  known,  is  now 
obviated  by  the  use  of  two  pistons,  set  at 
a  quarter  of  a  cycle,  or  90°  apart. 

In  order  to  obtain  two  separate  alterna- 
ting E.  M.  F.  's,  a  quarter  cycle  apart,  in 
two  separate  circuits,  either  two  separate 
windings  are  employed  on  a  single  arma- 
ture, or  two  separate  armatures  are  rigid- 
ly connected  and  driven  on  the  same 
shaft.  The  latter  method  is  represented 
in  Fig.  59,  where  a  750  KW.  or  1000  H.  P. 
diphase  generator  is  shown.  This  genera- 


Mtri/riPHASED  CUBBENTS. 


171 


tor  consists  of  two  complete  uniphase  gen- 
erators A  and  B;  I.  e.,  generators  of  the 
ordinary  single  alternating -current  type, 


FIG.  59.—  750-KiLOWATT   COLUMBIAN    EXPOSITION,  DIPHASE 
ALTERNATOR. 

rigidly  connected  together  in  such  a  man- 
ner that  the  armature  of  one  machine  is 
just  far  enough  ahead  to  produce  its  alter- 


172    ALTERNATING   ELECTRIC    CURRENTS. 

nating  E.  M.  F.  a  quarter  of  a  cycle  in  ad- 
vance of  that  of  the  other.  This  machine 
is  compound-wound,  supplying  its  field 
magnets  partly  from  the  commutator  (7, 
and  has  three  collector  rings  R,  R,  R, 
one  of  the  outside  rings  for  each  current 
and  the  middle  ring,  as  a  common  con- 


FIG.  go  —DIAGRAM  SHOWING  THE  Two  METHODS  OF  CONNECT- 
ING DIPHASE  ARMATURE  WINDINGS  THROUGH  COLLECTIVE 
RINGS  WITH  EXTERNAL  CIRCUITS. 

nection  for  both,   as  shown  in  Fig.  60. 
The  belt  tightening  handle  is  shown  at  H. 

Another  form  of  diphase  generator  is 
shown  in  Fig.  61.  Here  a  single  armature 
has  two  windings,  the  E.  M.  F.  in  one  of 
which  is  developed  a  quarter  of  a  cycle 


MULTIPHASE!)    CURRENTS. 


173 


before  the  other.  The  three  conductors 
A ,  B,  C,  carry  off  the  two  diphase  cur- 
rents, while  the  conductors  F,  F,  supply 


FIG.  61. — TOO- KILO  WATT  MULTIPHASE  GENERATOR. 

the  field  with  a  continuous  current.  The 
commutator  C,  supplies  current  to  the 
field  magnets. 


Another  form   of  diphase  generator  is 


174     ALTERNATING  ELECTRIC  CURRENTS. 


represented  in  Fig.  62.  Here  two  sepa- 
rate external  armatures  A  and  B,  do  not 
revolve,  while  within  them  revolves  the 
field  magnet  driven  by  a  pulley  P.  The 


FIG. 


-DiPHASE  ALTERNATING-CURRENT  GENERATOR. 


E.  M.  F.  in  one  armature,  say  A,  is  de- 
veloped a  quarter  of  a  cycle,  or  half  an 
alternation,  ahead  of  that  in  B. 


MULTIPHASED  CUEEENTS.      175 

The  circuits  of  such  a  diphase  generator 
require,  as  shown  in  Fig.  60,  either  three 
or  four  wires.  If  four  wires  are  em- 
ployed, the  two  separate  circuits  are  en- 
tirely distinct,  while  if  three  wires  are 
employed,  one  of  the  conductors  is  com- 
mon to  both  circuits. 


TWO  PHASE  ALTERNATING 
CURRENT  GENERATOR 


FlG.  63.—  DlPHASER  AN»  ITS  CIRCUIT. 

In  Fig.  63,  a  diphase  generator  or  di- 
phaseris  represented  at  A.  The  two  sep- 
arate currents,  generated  in  this  machine, 
are  led  to  the  transformers  T, ,  Tt ,  T3,  T4, 
through  the  three  wires  of  the  circuit. 
The  pressure  at  the  generator  brushes  is 
2000  volts  effective,  beween  C  and  D,  or 
between  D  and  E.  The  transformers  71 , 


176     ALTERNATING  ELECTHlC  CtJBRENTg, 

Tz  and  T4  are  connected  between  a  single 
pair  of  wires;  namely,  Z!, ,  between  C 
and  Z>,  T3  between  D  and  E,  and  jT4  be- 
tween D  and  ,£/,  so  that  only  one  current 
is  supplied  to  each  of  these  transformers. 
In  all  cases,  where  diphase  currents  are 
not  to  be  used  simultaneously  in  a  motor, 
they  are  separately  used  as  uniphase  cur- 
rents either  in  lamps  or  in  synchronous 
motors.  T4  is  a  transformer  on  one  of 
the  circuits  supplying  arc  lamps  L,L,  at  a 
pressure  of,  perhaps,  50  volts.  The  trans- 
former JJ ,  which  is  really  a  double  trans- 
former, half  between  the  wires  C  and  D, 
and  half  between  the  wires  D  and  E,  sup- 
plies in  its  secondary  circuits  G  and  //, 
diphase  currents  to  the  diphase  motor  M. 

A  triphase  generator  or  triphaser  is  a  gen- 
erator which  produces  three  separate  al- 
ternating E.  M.  F.'s  separated  from  each 


MULTIPHASED    CURRENTS. 


177 


other  by  one  third  of  a  cycle,  as  repre- 
sented in  Fig.  64.  Such  a  machine  is 
shown  in  Fig.  65.  Here  the  armature  has 
three  separate  windings  upon  it,  so  ar- 
ranged that  the  E.  M.  F.  's  generated  in 


FIG.  64.— DIAGRAM    REPRESENTING    PHASE    RELATION    OF 
TRIPHASE  WAVES  OF  E.  M.  F.  AND  CURRENT. 

them  succeed  each  other  by  one  third  of 
a  cycle.  Three  collector  rings  R1 ,  R2 ,  R3 , 
on  the  right  hand  armature  on  the  shaft, 
carry  off  the  current  as  shown  in  Fig.  66, 
to  three  wires,  AA\  BB\  CC\  each  of 


178  ALTERNATING  ELECTRIC  CURRENTS. 

which  serves  as  a  return  circuit  for  the 
other  two. 

The  motor  windings,  transformers,    or 
other  devices  are  connected  between  the 


FIG.  65.— 500-£iLOWATT  TRIPHASE  GENERATOR. 

wires  as  at  A1  B\  B1  C\  or  C1  A\  Triphasers 
possess  electrical  features  which  have 
gained  for  them  considerable  favor.  A 
triphaser  only  requires  three  wires  for  its 


MULTIPHASED    CURRENTS. 


179 


three  currents.     A  diphaser  requires  four 
wires  but  can  be  operated  with  three. 

Beside  the  diphase  and  triphase  gener- 
ators another  system  has  come  into  recent 


FIG. 66. — DIAGRAMS  REPRESENTING  CONNECTIONS  OF  TRIPHASE 
WINDINGS  WITH  THEIR  EXTERNAL  CIRCUITS. 

favor,  called  the  monocyclic  system.  The 
monocyclic  generator,  or  monocycler,  is 
primarily  a  uniphase  generator,  and  is  in- 
tended principally  for  the  delivery  of  or- 
dinary alternating  or  uniphase  currents, 


180     ALTERNATING  ELECTRIC  CURRENTS. 

over  a  system  of  electric  lighting  mains. 
In  order,  however,  to  supply  starting  al- 
ternating-current motors  wherever  they 
may  be  installed  in  the  system,  a  special 
series  of  coils,  of  smaller  size  and  cross - 
section,  is  placed  on  the  armature  so  as  to 
produce  a  small  E.  M.  F.  a  quarter  cycle 
out  of  step  with  the  main  uniphase  E.  M.  F. 
This  smaller  E.  M.  F.  is  connected  to  a 
third  collector  ring  on  a  special  circuit 
wire,  called  the  power  wire,  which  has  a 
smaller  cross -section  than  the  main  uni- 
phase wires,  and  is  led  only  to  where  the 
motors  are  to  be  used.  By  the  use  of  two 
transformers,  connected  with  the  power 
wi  e  and  the  main  wires,  triphase  E.  M. 
F.'s  are  produced  in  a  secondary  circuit 
for  the  operation  of  triphase  motors, 
while  between  the  main  wires  in  all 
other  parts  of  the  system,  ordinary  uni- 
phase E.  M.  F/s  are  maintained. 


MULTIPHASED    CUKKENTS. 


181 


A  form  of  belt-driven  150  KW.  monocy- 
clic  generator  is  represented  in  Fig.  67. 
Here  the  three  collector  rings  are  shown 


FIG.  67.— 150-KiLOWATT  MONOCYCLIC  GENERATOR. 

at  R,  R,  R,  and  the  commutator  C,  is  for 
the  compounding  of  the  field  magnets. 
Fig.  68  represents  the  armature  of  such  a 


182     ALTERNATING  ELECTRIC  CURRENTS. 

machine,  with  its  three  collector  rings  and 
its  commutator.  It  is  often  found  difficult 
to  determine,  from  the  appearance  of  such 
a  machine,  whether  it  is  of  the  monocyc- 


FIG.  68.— MONOCYCLIC  ARMATURE. 

lie,  diphase,  or  triphase  type,  but  a  close 
inspection  of  the  armature  will  usually 
indicate  that  the  main  coils  ZZ,  AA,  BB, 
are  larger  than  the  intermediate  coils  or 
lesser  coils  T,  T,  T,  T,  T,  T. 


CHAPTER  IX. 

MULTIPHASE  MOTORS. 

PRIOR  to  the  introduction  of  the  multi- 
phase machinery  there  were  but  two 
methods  whereby  electric  power  could  be 
commercially  transmitted  over  a  consider- 
able distance;  namely,  either  by  the  use 
of  continuous -current  motors,  or  by  the 
use  of  synchronous  alternating -current 
motors.  As  we  have  already  pointed  out, 
in  order  to  obtain  the  advantages  of  the 
electrical  transmission  of  power  it  is  nec- 
essary to  employ  a  high  pressure  on  the 
conducting  line  so  as  to  save  copper 
in  the  conductor.  While  this  is  possible 
by  the  use  of  continuous -current  motors, 
and,  in  point  of  fact,  has  been  employed, 
yet  the  presence  of  commutators,  which 


184    ALTEKNATING  ELECTRIC   CURRENTS. 

such  a  system  necessitates,  both  on  the 
generator  and  motor,  has  been  found,  in 
practice,  to  give  rise  to  no  little  risk  and 
trouble,  since  the  total  pressure  between 
the  lines,  being  thus  brought  directly  to  the 
opposite  sides  of  the  commutator,  should 
an  arc  discharge  occur  over  the  commu- 
tator, there  would  be  a  danger  of  its  de- 
struction. 

In  order  to  lessen  these  difficulties,  the 
plan  has  been  tried  of  distributing  the  line 
pressure  to  a  number  of  motors  all  rigidly 
connected  to  the  same  shaft,  and  traversed 
successively  by  the  driving  cr.Tent.  If, 
under  a  line  pressure  of  say  2503  volts,  five 
motors  were  so  coupled  together,  then 
each  motor  would  receive  a  pressure  of 
one  fifth  of  the  total,  or  500  volts.  Al- 
though this  device  reduces  the  pressure 
across  each  commutator,. yet  the  insulation 
of  each  machine  has  to  be  carefully  main- 


MULTIPHASE  MOTORS.  185 

tained,  since,  otherwise,  a  discharge  might 
take  place  through  the  commutators  lathe 
shjy£t,  under  the  whole  pressure  of  the  line, 
thus  disabling  the  plant.  Consequently, 
early  i  i  the  history  of  alternating  currents, 
appreciating  the  advantage  in  practice, 
arising  from  the  absence  of  a  commutator, 
the  uniphase  generator  and  motor  were 
connected,  by  means  of  conducting  lines, 
for  power  transmission.  To  a  certain  ex- 
tent this  combination  was  successful;  for, 
as  has  already  been  pointed  out,  beside 
the  advantage  of  collecting  rings  instead 
of  commutators,  the  system  possessed  a 
marked  advantage  from  the  ease  with 
which  the  pressure  could  be  varied  by  the 
aid  of  suitable  transformers.  When  the 
line  pressure  is  too  high  to  employ  safely 
at  the  brushes  of  generator  and  motor, 
these  latter  can  be  constructed  for  lower 
pressures  and  larger  currents,  and  then, 


186    ALTERNATING  ELECTRIC  OtJRRENTS. 

by  the  use  of  step -up  transformers  at  the 
generator  K  and  step -down  transformers  at 
the  motor,  all  the  advantages  of  high  pres- 
sure in  the  line,  and  low  pressure  at  the 
machinery,  can  be  secured,  without  great 
additional  risk  or  cost.  Such  a  system  of 
transmission,  however,  necessitates  the 
employment  of  the  uniphase  synchronous 
motor,  and  was,  therefore,  totally  unfitted 
to  cases  where  the  motor  had  to  be  fre- 
quently stopped  and  started. 

Happily  these  practical  difficulties  in 
the  commercial  transmission  of  power  have 
been  removed  by  the  introduction  of  multi- 
phase alternating- current  apparatus,  and 
while  it  is  true  that  the  use  of  such  ap- 
paratus necessitates  the  employment  of 
at  least  one  additional  conductor,  yet  the 
advantages  possessed  by  the  multiphase 
system  are  so  considerable,  that  even  al- 


MULTIPHASE  MOTORS.  187 

though  this  conductor  involved  extra  cost 
in  the  copper,  yet  the  advantages  obtained 
would  render  its  adoption  economical.  In 
point  of  fact,  however,  the  amount  of  cop- 
per actually  required  for  the  three -wire 
multiphase  system  is  one  fourth  less  than 
that  for  the  same  amount  of  power  by  the 
uniphase  system  employing  the  same 
pressure  in  the  line. 

As  at  present  employed  multiphase  cur- 
rents are  readily  divisible  into  diphase, 
triphase,  and  monocyclic.  Consequently, 
it  will  be  convenient  to  treat  motors 
under  the  same  general  heads.  In  point 
of  fact,however,  the  difference  between 
these  forms  of  motors  is  comparatively 
trivial.  A  diphase  motor  differs  from  a 
triphase  motor  mainly  in  the  fact  that  it 
has  two  circuits  in  its  fields  instead  of 
three. 


188    ALTERNATING  ELECTRIC  CURRENTS. 

In  order  to  understand  the  operation  of 
any  multiphase  motor,  we  will  consider  the 
effect  produced  on  a  suitable  field-wind- 
ing when  multiphase  currents  are  supplied 
to  it.  It  is  necessary  to  remember  that 
two  separate  alternating  currents,  flowing 
through  two  separate  circuits,  do  not  form 
a  diphase  system,  unless  the  two  currents 
differ  in  phase  by  a  quarter  cycle,  or  are 
90°  apart.  When  such  diphase  currents 
are  sent  through  properly  wound  field 
frames,  they  tend  to  produce  in  them  a 
magnetic  field  of  a  curious  character ;  name . 
ly,  the  poles  produced  do  not  only  alter, 
nate  in  direction  with  changes  in  the 
direction  of  the  current,  but  act  as  though 
the  field  rotated.  For  example,  if  in  Fig 
69,  we  consider  the  pair  of  coils  1,  3,  on 
the  opposite  sides  of  the  field  frame,  and 
suppose  that  a  single  uniphase  current  is 
supplied  to  them,  it  is  evident,  that  if  dur- 


MULTIPHASE  MOTOBS.  189 

ing  any  wave  of  current  the  pole  1  is  a 
north  pole  and  3,  a  south  pole,  then 
during  the  next  wave  of  reversed  current* 
these  poles  will  be  reversed  or  1  will  be 


FIG.  69.— DIAGRAMS  ILLUSTRATING  EFFECTIVE  ROTATION  OF 
A  DIPHASE  MAGNETIC  FIELD. 

south,  and  3,  north.  The  same  conditions 
will  be  maintained  in  the  adjacent  poles 
2  and  4,  which  are  alternately  north  and 
south,  and  south  and  north.  But  if  the 


190     ALTEBNATIKG  ELECTftlC  CUBBENTS* 

waves  of  current  through  C  and  D,  come 
half  an  alternation  later  than  the  waves  in 
A  and  B,  we  obtain  a  series  of  conditions 
represented;  namely, 

(A)  1  is  north,  3  is  south,  while  4  and 
2  are  in  transition,  there  being  no  current 
in  them  at  that  instant. 

(B)  In  the  next  quarter  cycle,  4  and  2 
are  now  active,  while  3  and  1  are  in  tran- 
sition. 

(C)  At  the  next  quarter  cycle  3  and  1  have 
again  come  into  action  in  the   opposite 
direction,  while  2  and  4  are  in  transition, 
and  finally: 

(D)  In  the  fourth  quarter  of  the  cycle,  1 
and  3  are  in  transition,  while  4  and  2  are 
active.     If,  now,  we  examine  these  figures 
we  shall  see  that  the  N.  and  S.  poles  have 
steadily  progressed  around  the  field  frame 
in  the  direction  of  the  hands  of  a  clock,  so 
that,  although  alternating  currents  have 


MULTIPHASE    MOTOES.  191 

been  employed,  yet  by  reason  of  their 
proper  phase  difference  in  the  two  separate 
circuits,  their  effect  has  been  to  cause  the 
magnetic   field  to   rotate.     If  a  compass 
needle  were  introduced  into  the  middle  of 
the  field  frame,  it  would,  if  left  free  to  spin 
around  the  axis,  rotate  about  that  axis  at 
the  rotary  speed  of  the  field;  namely,  one 
revolution  per  cycle.   Such  a  rotating  com- 
pass needle  may  be  considered  as  a  small 
armature  capable  of  acting  as  a  motor.     A 
piece  of  soft  iron  pivoted  upon  an  axis  at 
the  centre  will  revolve  in  the  same  way. 
In  practice  it  is  usual  to  construct  a  lamin- 
ated armature  core,  like  that  of  a  contin- 
uous-current   motor,  wound  with  closed 
coils  or  closed  loops,  so  as  to  induce  power- 
ful currents  in  these  coils  by  the  rotation 
of  the  magnetic  flux   through  them,  and 
thus  develop  a  powerful  magnetic  attrac- 
tion between  the  revolving  magnetic  field 


192     ALTERNATING  ELECTRIC  CURRENTS. 

and  these  currents .   Such  mo  tors  are  there  - 
fore  sometimes  called  induction  motors. 

In  order  to  reverse  the  direction  of  a 
polyphase  motor  it  is  only  necessary  to  re- 
verse the  direction  of  one  of  the  windings 
on  the  motor,  so  as  to  reverse  one  of  the 
pairs  of  poles,  when  the  field  will  rotate  in 
the  opposite  direction.  With  the  appa- 
ratus actually  employed  a  switch  is 
arranged,  so  that,  by  its  motion,  one  of  the 
field  windings  is  reversed. 

A  triphase  motor  differs  from  a  diphase 
motor  only  in  that  its  field  windings  con- 
tain either  six  coils,  or  some  multiple 
of  three,  instead  of  four  coils  or  some  mul- 
tiple of  four.  The  effect  of  the  current 
waves  succeeding  each  other  in  the  differ- 
ent windings,  by  one  third  of  a  cycle,  pro- 
duces a  continuously  rotating  field. 


MULTIPHASE  MOTOES. 


193 


Fig.  70  represents  a  15  H.  P.  diphase 
motor.  FF  is  the  field  frame  of  laminated 
iron  with  suitable  windings  inside  to  pro- 


FIG.  70.— FIFTEEN-  HORSE-POWER  BIPHASE  MOTOR. 

duce  the  revolving  field,  within  which  the 
armature  rotates   driving   the  pulley  P. 


194     ALTERNATING  ELECTRIC  CUEEENTS. 

It  is  important  to  observe  that  in  syn- 
chronous motors,  the  field  frame  need  not 
be  laminated,  since  the  field  poles  do  not 
change  polarity,  being  excited  by  a  con- 
tinuous current,  but  in  multiphase  mo- 
tors, since  the  field  magnets  are  excited 
by  alternating  currents,  it  is  important 
that  the  iron  be  laminated,  in  the  frame 
as  well  as  in  the  armature,  since,  other- 
wise, loss  of  power  and  injurious  heating 
would  occur. 

Fig.  71  shows  a  form  of  triphase  motor 
for  71-2  horse-power.  The  three  con- 
ducting wires  are  led  through  the  winding 
of  the  field  to  the  terminals  A,  B,  C,  and 
the  armature  shaft  has  a  series  of  con- 
tacts (7,  which  is  not  a  commutator,  al- 
though somewhat  resembling  one  in  ap- 
pearance. When  the  handle  H,  is  in 
the  position  shown,  certain  resistances 


MULTIPHASE  MOTORS. 


195 


are  included  in  the  circuit  of  the  armature 
windings,  so  as  to  enable  the  motor  to 
start  from  rest.  It  is  found,  that  if  the 
full  pressure  be  supplied  to  the  field  of 


FIG.  71.— -TRIPHASE  INDUCTION  MOTOR,  7%  H.P. 

the  motor  with  the  armature  in  its  ordi- 
nary short -circuited  condition,  such  pow- 
erful currents  are  induced  in  the  armature 


196     ALTEENATING  ELECTBIC  CUEEENTS. 

as  to  weaken  its  starting  power.  By  the 
insertion  of  extra  resistance,  however, 
these  currents  can  be  reduced  to  the 
proper  strength  in  the  armature  circuits 
to  obtain  a  powerful  starting  power  or 
torque,  and,  when  the  machine  has  attained 
full  speed,  the  handle  is  pushed  in  toward 
the  field  frame,  thereby  sliding  the  con- 
tact ring  C,  into  the  strong  clips  of  C1, 
short-circuiting  the  extra  resistance,  and 
cutting  it  out  of  circuit.  The  size  of  this 
motor  is  indicated  by  a  foot-rule  RR, 
shown  at  its  base. 

Fig.  72  represents  a  similar  triphase 
motor  for  125  H.  P.  The  three  terminals 
of  the  field  winding  are  shown  at  the  top 
of  the  frame  F,  F,  F,  F;  within  revolves 
the  armature  A,  A,  A.  As  in  the  last  case, 
the  handle  H,  when  the  motor  has  been 
brought  up  to  speed,  throws  forward  a 


MULTIPHASE  MOTORS. 


197 


collar  K,  into  a  receptacle,  thus  cutting 
the  starting  resistance  out  of  the  circuit  of 


FIG.  72. — 125-HoRSE-PowEB  INDUCTION  MOTOR. 

the  armature  coils.  It  will  be  seen  that 
these  triphase  motors  are  very  simple  in 
appearance,  have  self-oiling  bearings,  and, 


198     ALTERNATING    ELECTRIC   CURRENTS. 

having  no  commutator,  require  the  mini- 
mum of  att  ention. 

Another  form  of  small  induction  motor 
is  represented  in  Fig.  73.     This  is  a  tri- 


FIG.  73.— MONOCYCLIC  MOTOR. 

phase    motor  frequently    operated  on   a 
monocyclic  circuit. 


MULTIPHASE   MOTORS. 


199 


Figs.  74  and  75  show  a  form  of  diphase 
motor,  with  front  and  rear  view.  The 
three  collector  rings  R\  R\  R  ,  are  em- 


pIG>  74.— BIPHASE  MOTOR. 

ployed  for  the  purpose  of  inserting  resist- 
ance in  the  armature  circuits  under  the 
control  of  the  handle  H,  which  is  only 


200     ALTEENATING  ELECTRIC   CUEEENTS. 

employed  in  starting  the  motor.     As  soon 
as  full  speed  is  reached,  the  additional  re- 
sistance is  entirely  cut  out  of  circuit. 
The  interior  of  the  field  frame  for  this 


FIG.  75— BIPHASE  MOTOR. 

motor  is  represented  in  Fig.  76.  It  will 
be  seen  that  there  are  two  separate  field 
frames  placed  side  by  side,  but  differing 


MULTIPHASE  MOTORS.  201 

in  relative  position.  One  of  the  two  di- 
phase  currents  supplies  the  series  A,  B,  C, 
and  the  other  diphase  current,  the  series 
A l ,  Bl ,  C1 .  Under  these  conditions,  al- 


FIG.  76.— MOTOR  FIELD. 

though  no  rotating  magnetic  field  is  pro- 
duced, yet  by  the  effect  of  these  alterna- 
ting magnetic  poles  upon  the  armature,  a 
rotating  magnetic  field  is  developed  upon 


202     ALTERNATING   ELECTRIC   CURRENTS. 


it.  The  armature  is  represented  in  Fig. 
77.  At  A,  the  core  is  shown,  consisting 
of  two  separate  halves  ZTand  H\,  each 
revolving  under  one  series  of  field  mag- 
nets in  the  field  frame.  The  appearance 


FIG.  77. —MOTOR  ARMATURES. 

of  the  armature  after  winding  is  shown  at 
B,  where  the  wire  occupies  the  grooves 
between  the  iron  teeth  on  the  armature 
surface.  The  winding  is  carried  com- 
pletely across  the  double  armature,  so 


MULTIPHASE  MOTORS,  203 

that  the  currents  produced  in  the  winding 
by  one  series  of  field  poles  react  upon  the 
neighboring  series.  This  motor  is  de- 
signed for  a  frequency  of  about  130  cycles 
per  second.  Triphase  and  diphase  mo- 
tors, while  they  can  be  designed  for  other 
frequencies,  are  more  commonly  em- 
ployed at  a  frequency  of  60  or  30  cycles 
per  second. 

The  practical  trend  at  the  present  time 
is  toward  the  introduction  of  multiphase 
systems  for  the  transmission  of  electric 
power.  This  tendency  has  resulted  from 
the  great  flexibility  possessed  by  multi- 
phase systems. 

Such,  in  brief,  is  a  description  of  the 
more  important  commercial  applications 
of  alternating -current  apparatus.  When 
we  consider  that  the  developments  in 
this  latest  field  of  electrical  improve- 


204     ALTERNATING  ELECTKIC  CURKENTS. 

ment  have  occurred  practically  within  less 
than  a  decade,  we  cannot  but  believe  that 
the  next  decade  will  witness  even  still 
greater  improvements  in  this  rapidly- 
advancing  art. 

THE  END. 


INDEX. 


A 

Action  of  Biphase  Armature  Windings,  172. 
Active  Conductor,  Magnetic  Properties  of,  30,  31. 
Activities,  Electrical,  Examples  of,  89-91. 
Activity,  Apparent,  in  Alternating-Current  Circuit,  94. 

• ,  Influence  of  Eeactance  on,  96-98. 

—  of  Motor,  Definition  of,  85. 

—  of  Source,  Methods  of  Increasing,  87,  88. 
Adjustable  Resistance,  72. 

Advantages  Possessed  by  Multiphase  Motors,  183-186. 
Air  Insulation  of  Transformer,  126. 
Alternating-Current  Arc  Lamp,  149-151. 

-  Circuit,  15. 

-  Circuit,  Power  Factor  of,  94,  95. 

Circuits,    Advantages    of   in  Long-Distance 

Transmission,  107,  108. 

-  Cycle,  9. 

—  Dynamo-Electric  Machine,  60. 
— ,  Electrical  Activity  in,  91,  92,  93. 
—  Electromagnet,  45. 

Fan  Motors,  164-166. 

Flow,  Curve  of,  16. 


206     ALTERNATING  ELECTRIC  CURRENTS. 

Alternating -Current,  Period  of,  9. 

-  Transformer,    Insulation   Between    Primary 

and  Secondary  Circuits,  Necessity  for,  116, 
117. 

—  Transformer,  Operation  of,  110,  111. 

—  Transformer,  Self- Regulating  Properties  Pos- 

sessed by,  111,  112. 
Transformer,  Simple  Form  of,  109. 

—  Transformers,  99-132. 

Transformers,  Primary  and  Secondary  Con- 
nections of,  100. 

—  Transformers,  Ratio  of  Primary  and  Second- 

ary Pressures  in,  112,  113. 
Alternating  Currents,  Dangers  of,  to  Life,  50 

-  E.  M.  F.,  23,  26. 

—  Electric  Current,  6,  23. 

Electric    Currents,  Dangers  Possessed   by, 

38,  39. 
Electromotive  Forces    and    Currents,  21-25. 

—  Tidal  Currents,  6. 

-  Watermotive  Force,  26. 
Alternation,  Frequency  of,  7. 
Alternations,  Semi-period  of,  9. 

Alternator,  Compound-Wound,  Separately-Excited, 

Illustration  of,  78,  79. 
,  Separately-Excited,  67,  69. 


207 

Alternators,  26,  60. 

,  Self-Excited,  63. 

,  Self-Excited,  an  Example  of,  76. 

,  Self-Regulating  Compound- Wound,  70,  71. 

— ,  Separately-Excited,  63. 
Ampere,  14. 

Apparatus,  Multiphase,  167. 
Apparent  Activity  in  Alternating-Current  Circuit,  94. 

—  Resistance,  37. 

Arc  Lamp,  Electrical  Activity  in,  90. 
Arc  Lamps,  Alternating-Current,  149-151. 
Armature,  Monocyclic,  182. 

—  of  Dynamo-Electric  Machine,  58,  59. 
Automatic  Choking  Effects  of  Alternating  Currents, 

38,  39. 
Regulation  of  Dynamos,  70. 


B 


Bipolar  Continuous- Current  Generator,  64. 

Dynamo,  Definition  of,  65. 

Field,  60. 

Block,  Fuse,  119. 

Blowing  of  Fuse  Wires,  121. 

Brilliancy  of  Lamp  Filament,  135,  136. 


208      ALTERNATING  ELECTKIC  CURRENTS. 
C 

C.  E.  M.  F.  of  Self-induction,  33,  34. 
Cell,  Terminals  of,  25. 

— ,  Voltaic,  23. 
Central  Electric  Station,  57,  58. 

—  During  Full  Load,  Peculiarities  Presented  by, 

81-83. 

Choking  Coil,  Use  of,  in  Series  Alternating-Current 
Circuit  for  Incandescent  Lamps,  147,  148. 

— ,  Effect  of  Coil,  37,  38. 

— ,  Effects  of  Alternating  Currents,  38,  39. 
Circuit,  Alternating-Current,  15. 

— ,  Closed,  23. 

— ,  Completed,  23. 

— ,  Connections  of  Biphase  Generator,  175,  176. 

— ,  Definition  of,  22. 

— ,  Impedance  of,  35,  36. 

— ,  Inductionless,  95,  96. 

Circumstances  Affecting  Value  of  Impedance,  37,  38. 
Closed  Circuit,  23. 
Coil,  Choking  Effect  of,  37,  38. 

— ,  Primary,  of  Alternating-Current  Transform- 
er, 109. 
,  Reactive,  42. 


INDEX.  209 

Coil,  Secondary,  of  Alternating-Current  Trans- 
former, 110. 

Collector  Kings  of  Alternators,  61. 

Commercial  Efficiency  of  Incandescent  Lamp,  137. 

Commutator  of  Dynamo-Electric  Machine,  60. 

Compensator  or  Choking  Coil  for  Alternating-Cur- 
rent Arc  Lamp,  152. 

Completed  Circuit,  23. 

Composite  Dynamo-Electric  Machine,  70. 

Compound-Wound  Dynamo-Electric  Machine,  70. 

—  Machines,  75. 
Conducting  Paths,  21. 

Conjoined  Eelations  of  Flux  and  E.  M.  F.,  33,  34. 
Continuous  E.  M.  F.,  23. 

—  Current  Dynamos,  60. 
—  Electric  Currents,  6. 

Convention  as  to  Assumed  Direction  of  Magnetic 

Flux  in  a  Circuit,  32. 
Core,  Laminated,  of  Transformer,  108. 
Coulomb-per-second,  14. 
Counter  E.  M.  F.,  33,  34. 
Current,  Continuous,  23. 
Currents,  Alternating-Electric,  6. 

— ,  Alternating-Electric,  Dangers  Possessed  by, 

38,  39. 
,  Alternating-Tidal,  6, 


210     ALTERNATING  ELECTRIC  CURRENTS. 

Currents,  Multiphase,  167-182. 

Curve  of  Alternating-Current  Flow,  16. 

Cycle  of  Alternating-Current,  9. 

-  of  Eiver  Flow,  8. 

D 

Daniell  Gravity  Voltaic  Cell,  24. 

Definition  of  Alternating-Current  Transformer,  108. 

of  Bipolar  Dynamo,  65. 

-of  C.E.  M.  F.,  33,  34. 
— of  C.  E.  M.  F.  of  Self-Induction,  33,  34. 

—  of  Circuit,  22. 
of  Machine,  55,  56. 

—  of  Phase  Difference,  94,  95. 
— : —  of  Torque  of  Motor,  169. 

-  of  Watt,  87. 

Devices,  Translating  or  Eeceptive,  21. 
Dimmer,  Theatre,  41,  42. 

Diphase  Alternating  Currents,  Relations  Between, 
168,  169. 

—  Alternator,  170,  171. 

Armature  Windings,  Action  of,  172. 

Generator,  169. 

Generator,  Circuit  Connections  of,  175,  176. 

—  Magnetic  Field,  Diagram  Illustrating  Rota- 

tion of,  189. 


INDEX.  211 

Diphase  Motor,  Illustration  of,  193,  194. 

—  Motors,  160,  167. 

Motors,  Illustrations  of,  199,  200. 

Diphaser,  Definition  of,  175. 
Double  Winding  on  Field  Magnets,  70. 
Driven  Machinery,  58. 
Driving  Machinery,  .58. 
Drop  of  Hydraulic  Pressure,  102. 
Drop  of  Pressure,  Effect  of  Length  of  Circuit  on, 
104,  105. 

of  Pressure  in  Circuits,  101-103. 

of  Pressure  in  Electric  Circuits,  Illustration 

of,  101-103. 

-  of  Voltage,  101-103. 
Dynamo,  Quadripolar,  Definition  of,  66. 

— ,  Keversibility  of,  153. 
Dynamo-Electric  Machine,  Armature  of,  58,  59. 

—  Commutator,  60. 

— ,  Compound- Wound,  70. 

— ,  Field  Magnets  of,  59, 
Dynamos,  Continuous-Current,  60. 

E 

E.  M.  F.,  Alternating,  23. 

and  Flux,  Conjoined  Relations  of,  33,  34, 


212     ALTERNATING  ELECTRIC  CURRENTS. 

E.  M.  F.,  Continuous,  23. 

— ,  Effective,  Definition  of,  51. 
— ,  Impressed,  Definition  of,  33,  34. 
— ,  Meaning  of,  22. 
— ,  Kesultant,  96. 
— ,  Unidirectional,  23. 
E.  M.  F.  's,  Induced,  33,  34. 
Effective  Current  Strength,  Definition  of,  51. 

-  E.  M.  F.,  Definition  of,  51. 
Effects,    Electroplating,   Produced    by  Alternating 

Currents,  47. 

,  Electroplating,  Produced  by  Continuous  Cur- 
rents, 56. 

,  Heating,  produced  by  Continuous  and  Alter- 
nating Currents,  58. 
— ,  Physiological,  48,  49. 
— ,  Physiological,  of  Electrical  Currents,  50. 

,  Tesla,  7. 

Efficiency  of  Electric  Motor,  155. 

—  of  Incandescent  Lamp,  Influence  of  Tempera 

ture  on,  138,  139. 
Electric  Current,  Alternating,  23. 

—  Currents,  Continuous,  6. 

—  Lamps,  133-152. 
— Motors,  153-166, 


INDEX.  213 

Electric  Motors,  Intake  of,  155. 
—  Resistance,  27. 

—  Resistance,  Unit  of,  27. 

Electrical  Activities  of  Incandescent  Lamp,  89,  90. 

—  Activity  of  Alternating  Current  Dependent 

on  Phase  Relations,  92-94. 
Activity  in  Arc  Lamp,  90. 

—  Activity  of  Railway  Generator,  90,  91. 

—  Transmission  of  Power,  155-157. 

—  Transmission  of  Power,   Use  of  Continuous 
Currents  and    Alternating   Currents    for, 
156-158. 
Electricity  and  Ether,  Action  Between,  18-20. 

— ,  General  Nature  of,  17,  18. 
Electrocution,  49. 
Electromagnet,  45. 

— ,  Alternating-Current,  45. 
Electromotive  Force,  Abbreviation  for,  22. 
Electroplating,  46. 

— ,  Effects  Produced  by  Alternating  Currents,  47. 

— ,  Effects  Produced  by  Continuous  Currents,  46. 
Ether,  18. 

—  Streamings,  31. 

Examples  of  Electrical  Activity,  89-91, 


214     ALTERNATING  ELECTRIC  CURRENTS. 

F 

Fan  Motor,  Alternating-Current,  164-166. 
Filament,  Incandescence  of,  134. 

—  of  Incandescent  Lamp,  133,  134. 
Field,  Bipolar,  60. 

— ,  Magnetic,  Definition  of,  32. 
Magnets  of  Dynamo-Electric  Machine,  59. 

-  Winding  of  Triphase  Motor,  192,  193. 
Flux  and  E.  M.  F.,  Conjoined  Kelations  of,  33,  34. 

— ,  Magnetic,  Kate-of-Change  of,  35. 
Foot- Pound,  Definition  of,  84. 

-  Per-Second,  85. 

Force,  Alternating  E.  M.  F.,  26. 

— ,  Watermotive,  26. 
Forces,  Magnetomotive,  59. 

Frequency  in  Alternating-Current  Circuit,  Influence 
of,  on  Steadiness  of  Lamps,  135,  136. 

,  Influence  of,  on  Impedance,  37,  38. 

of  Alternatiug-Eleclric  Currents  in  Light,  8. 

of  Alternation,  7,  9. 

—  of  Telephonic  Circuits,  7. 
Fuse-Block,  119. 

Fuse-Block  of  Transformer,  120,  121. 
Fuse  Wires,  Blowing  of,  121, 


INDEX,  215 

G 

General  Nature  of  Electricity,  17,  18. 
Generator,  Bipolar  Continuous-Current,  64. 
General    Construction    of    Incandescent     Electric 

Lamp,  133,  134. 
Generator,  Di phase,  169. 

— ,  Monocyclic,  179,181. 
Generator,  Quadripolar  Continuous-Current,  65. 

— ,  Triphase,  176,  177. 
Generators,  Multipolar,  68. 

H 

Heating   Effects    by  Continuous    and   Alternating 

Currents,  48. 
High- Frequency  Currents,  Tesla's  experiments  on, 

49,  50. 

High-Pressure  Incandescent  Electric  Lamps,  138,139. 
Horse-power  and  Kilowatt,  Kelation  Between,  88,  89. 

— ,  Definition  of,  85. 
Hydraulic  Pressure,  Illustration  of  Drop  of,  102. 

I 

Illustration  of  Diphase  Motor,   193,  194. 
of  Triphase  Induction  Motor,  195,196. 


216     ALTERNATING  ELECTRIC  CURRENTS. 

'  Impedance,  Circumstances  Affecting  Value  of  ,37,38. 
— ,  How  Affected  by  Frequency,  37,   38. 

of  Circuit,   Influence  of  Iron  upon  Value  of, 

37,   38. 

of  Circuits,  35,  36. 

Impressed  E.  M.  F.,  Definition  of,  33,  31. 
Incandescence  of  Lamp  Filament.  134. 
Incandescent  Electric  Lamp,  Varieties  of,  141-143. 
Incandescent  Filament,  Brilliancy  of,  135,  136. 

—  Lamp  Base,  Varieties  of,  141-143. 

—  Lamp,  Electrical  Activities  of,  89,  90. 

—  Lamp,  Filament  of,  133,  134. 

—  Lamp  on   Alternating-Current  Circuit,  Flick- 

ering in,  135,  136. 
-  Lamp,  Parallel  Connection  of,  144-146. 

—  Lamp,  Socket  of,  140. 

—  Lamp,  Temperature  of,  140 

—  Lamp,  Three-Wire  System  of  Distribution  of, 

146. 

—  Lamps  High-Pressure,  138,  139. 

—  Lamps,  Low-Pressure,  139. 

—  Lamps,  Series  Distribution  of,  146,  147. 

—  Lamps,    Two- Wire  System  of  Distribution, 

141,  145. 
Indoor  Type  of  Transformer,  127,  128. 


INDEX.  217 

Induced  E.  M.  F.'s,  33,  34. 

Induction  Motor,  Starting  [Resistance  of,  197,  198. 

-  Motors,  191,  192. 
Inductionless  Circuit,  95,  96. 
Intake  of  Electric  Motors,  155. 

Intensity  of  C.  E.  M.  F.,    Circumstances  Affecting 

Value  of,  35. 
Iron,  Effect  of,  on  Impedance  of  Circuit,  37,  38. 

-  Influence  of,  on  Keactance  of  Circuit,  37,  38. 

K 

Kilowatt,  Definition  of,  88. 


Lamp,  Incandescent,  Commercial  Efficiency  of,  137. 
— ,  Incandescent  Electric,  General  Construction 

of,. 133,  134. 

— ,  Incandescent,  Life  of,  138. 
Lamps,  Electric,  133-152. 
Length  of  Circuit,  Effect  of,  on  Drop  in  Pressure, 

104,  105. 

Life  of  Incandescent  Lamp,  138. 
Light,    Frequency   of  Alternating- Electric  Current 

in,  8. 


218      ALTERNATING  ELECTRIC  CURRENTS. 

Long-Distance      Transmission,      Advantages     of 

Alternating  Current  on,  107, 108. 
Low-Pressure  Incandescent  Lamps,  138,  139. 

M 

M.  M.  F.'s,  59. 

Machine,  Definition  of,  55,  56. 

Machinery,  Driven,  58. 

— ,  Driving,  58. 
Magnet,  Electro,  45. 
Magnetic  Flux,  Convention  as  to  Assumed  Direction 

of,  in  a  Circuit,  32. 

-  Properties  of  Active  Conductor,  30,  31. 
Magnetism,  Definition  of,  31. 
Magnetomotive  Forces,  59. 
Method  of  Connecting  Transformer  Secondary  Coils 

for  50  or  100  volts,  123,  124. 
Monocycler,  179. 
Monocyclic  Generator,  179,  181. 

—  Motor,  Illustration  of,  198. 

—  System,  179. 

Motor,  Definition  of  Torque  of,  169. 

— ,  Multiphase,  167. 
Motors,  Diphase,  160. 

— ,  Electric  Efficiency  of,  155. 

— ,  Electric  Output  of,  155. 


INDEX.  219 

Motors,  Induction,  191,  192. 
,  Multiphase,  183-203. 

— ,  Multiphase,       Advantages       Possessed    by, 
183-186. 

Synchronous  Electric,  158,  159. 

Triphase,  167. 

Multiphase  Apparatus,  167. 
Current,  157-182. 

—  Generator,  173. 

-  Motor,  167. 
Multiphase  Motors,  183-203. 

Motors,  Advantages    Possessed    by,  183-186. 

Motors,    Eotary    Properties     of    Field    in, 

188-191. 
Multipolar  Generator,  68. 

Generator,  Necessity    for   Even  Number   of 

Poles  in,  68. 


N 


Negative  Pole  of  Voltaic  Cell,  25. 
Non-Luminous  Radiation  of  Lamp,  137. 
Number  of  Turns  in  a  Circuit,  Effect  of,  on  Impe- 
dance, 373,  38. 


220     ALTEEtfAf  IJfG  ELECTBIC  CUKKENTS. 

0 

Oersted's  Discovery,  31. 

Ohm,  Definition  of,   27. 

Ohmic  Kesistance,  39. 

Ohm's  Law,  28. 

Oil-Insulated  Transformer,  117. 

Operation  of  Alternating  Current  Transformer,  110, 

111. 

Out-door  Type  of  Transformer,  125. 
Output  of  Electric  Motors,  155. 


Period  of  Alternating  Current,  9. 

—  of  Eiver,  8. 
Phase  Difference,  Definition  of,  94,  95. 

Difference  of  E.  M.  F.  and  Current  Effect  of, 

on  Electrical  Activity,  94. 
Physiological  Effects,  48,  49. 
Pole,  Negative,  of  Voltaic  Cell,  25. 
— ,  Positive,  of  Voltaic  Cell,  25. 
Positive  Pole  of  Voltaic  Cell,  25. 
Power,  Electric,  81-98. 

,  Electric,  Transmission  of,  155,   156. 

Factor  of  Alternating-Current  Circuit,  94. 


INDEX.  221 

Power  Wire  of  Monocyclic  System,  180. 

Pressure,  Drop  of,  in  Circuits,  101-103. 

Primary    and  Secondary  Circuits  of  Transformer, 

Ratio  of  Pressure  in,  112, 113. 
Primary  Coil  of  Alternating-Current  Transformer, 

109. 
Primary  Power-Factor  of  Transformer,  126,  127. 

Q 

Quadripolar  Continuous-Current  Generator,  65. 
Quadripolar  Dynamo,  Definition  of,  66. 

E 

Kail  way  Generator,  Electrical  Activity  of,  90,  91. 

Kate-of-Change  of  Flux,  3. 

Eeactance,  30. 

,  Influence  of,  on  Activity,  96-98. 

of  Circuit,  Influence  of  Iron  on,  37,  38. 

Reactive  Coil.,  42. 

Coil  for  Series  Alternating-Current  Incandes- 
cent Lamp,  150. 

Coil  or  Compensator  for  Alternating-Current 

Arc  Lamps,  152. 

Receptive  or  Translating  Devices,  21. 


222      ALTEB.H  ATTNG  ELECTEIC  CUEEENTS. 

Regulation,  Automatic,  of  Dynamos,  70. 

Relations   Between  Diphase  Alternating-Currents, 

168,  169. 
Resistance,  Adjustable,  72. 

— ,  Apparent,  37. 

— ,  Electric,  27. 
-  of  Alternating  Currents,  29,  30. 

—  of  Continuous-Current  Circuits,  Circumstances 
Influencing  Value  of,  29,  30. 

— ,  Ohmic,  39. 
ResultantE.  M.  F.,  96. 
Reversibility  of  Dynamo,  153. 
Rheostats,  72. 

Rings,  Collector,  of  Alternators,  61. 
River  Flow,  Cycle  of,  8. 

— ,  Period  of,  8. 
Rotation  of  Diphase  Field,  Diagram  of,  189. 

s 

Secondary  Coil  of  Alternating-Current  Transformer, 

110. 
Self- Excited  Alternators,  63. 

Machines,  Class  of,  74,  75. 

Self- Regulating  Compound- Wound  Alternator, 70,  71. 
Self- Regulation  of  Alternating- Current  Transformer, 

111,  112. 


INDEX.  223 

Semi-Period  of  Alternations,  9. 
Separately-Excited  Alternators,  63,  67,  69. 

Compound-Wound  Alternator,  71,  72. 

Machines,  Class  of,  74. 

Series  Distribution  of  Incandescent  Lamps,  146,  147. 

Socket  of  Incandescent  Lamp,  140. 

Source,  Electric,  Definition  of,  21. 

Starting  Eesistauce  of  Induction  Motor,  197,  198. 

Step-down  Transformers,  114. 

Step-up  Transformers,  114. 

Streaming  of  the  Ether,  31. 

Sub-Station  Transformer,  130-132. 

Synchronous  Electric  Motors,  158,  159. 

System,  Monocyclic,  179. 


Telephonic  Alternating  Currents,  6. 

— ,  Circuits,  Frequency  of,  7. 
Temperature  Influence  of  an  Incandescent  Lamp, 

138,  139. 

Terminals  of  Voltaic  Cell,  25. 
Tesla  Effects,  7 . 

— ,  Experiments    of,   on    High-Frequency  Cur- 
rents, 49,  50. 

The  Temperature  of  Incandescent  Lamps,  140. 
Theatre  Dimmer,  41,  42. 


224      ALTERNATING  ELECTRIC  CURRENTS. 

Theatre  Dimmers,  Varieties  of,  42-44. 

Three-Wire  System  of  Distribution  of  Incandescent 

Lamps,  146» 
Tidal  Flow  of  River,  11, 

Level  of  River,  12. 

Torque  of  Motor,  Definition  of,  169. 
Transformer,  Air  Insulation  of,  126. 

— ,  Alternating-Current,  Definition  of,  108. 

— ,  Fuse-box  of,  120, 121. 

— ,  Indoor  Type  of,  127,  128. 

— ,  Oil-Insulated,  117. 

— ,  Out-door  Type  of,  125. 

— ,  Primary  Power  Factor  of,  126,  127. 
Transformer,  Sub-Station  Type  of,  130-132. 
Transformers,  Alternating-Current,  09-132. 

— ,  Laminated  Iron  Core  of,  115,  116. 

— ,  Step-up,  114. 

— ,  Use  of,  in  Electric  Transmission  of  Power, 

159-163. 

Translating  or  Receptive  Devices,  21 . 
Triphase  Currents,  Phase  Relations  Between,  177. 
—  Generator,  176,  177. 

—  Generator,   Illustration  of,  178. 

-  Induction  Motor,  Illustration  of,  195, 196. 

-  Motor,  Field  Windings  in,  192,  193. 
Motors,   167. 


INDEX.  225 

Work  Winding,  Diagram  Representing  Connections 

of,  179. 

Triphaser,  Definition  of,   176,  177. 
Two-Wire  System  of  Distribution   of  Incandescent 

Lamps,  144,  145. 

u 

"Unidirectional  E.  M.  F.,  23. 
Uniphase  Alternators,  53-80. 
Unit  of  Activity,  85. 

of  Electrical  Flow,  13. 

—  of  Electric  Resistance,  27. 
of  Work,  84. 


Varieties  of  Incandescent  Lamps,  141-143. 
Volt-Coulomb-per-second,  or  Watt,  Definition  of,  87. 
Voltage,  Drop  of,  101-103. 
Voltaic  Cell,  23. 


Watt,  Definition  of,  87. 

Winding,  Triphase,  Diagram  Connections  of,  179. 
Wire,  Power,  of  Monocyclic  System,  180, 
Work,  Unit  of,  84. 


Elementary 
Electro  -  Technical  Series. 

BY 

EDWIN  J,  HOUSTON,  Ph.D,  and  A,  E,  KENNELLY,  D.Sc, 


Alternating  Electric  Currents,  Electric  Incandescent  Light- 
Electric  Heating,  ing, 

Electromagnetism,  Electric  Motors, 

Electricity  in  Electro-Thera-  Electric  Street  Railways, 

peutics,  Electric  Telephony, 

Electric  Arc  Lighting,  Electric  Telegraphy. 


Cloth,  profusely  illustrated.  Price,  $1.OO  per  volume. 


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cerning the  several  departments  of  electrical  science  treated, 
and  the  reputation  of  the  authors,  and  their  recognized  ability 
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